Process for making thin-wet absorbent foam materials for aqueous body fluids

ABSTRACT

Relatively thin, collapsed, i.e. unexpanded, polymeric foam materials that, upon contact with aqueous body fluids, expand and absorb such fluids, are disclosed. A process for consistently obtaining such relatively thin, collapsed polymeric foam materials by polymerizing a specific type of water-in-oil emulsion, commonly known as High Internal Phase Emulsions or &#34;HIPE&#34;, is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 07/989,270, filed on Dec.11, 1992, now U.S. Pat. No. 5,387,207 which is a continuation-in-part ofU.S. application Ser. Nos. 07/743,838 now abandoned, 07/743,839 now U.S.Pat. No. 5,260,345 and 07/743,951, now U.S. Pat. No. 5,352,711 all filedAug. 12, 1991, and U.S. application Ser. No. 07/935,935, filed Aug. 27,1992 now U.S. Pat. No. 5,198,472, which is a divisional of U.S.application Ser. No. 07/830,159, filed Feb. 3, 1992 (now U.S. Pat. No.5,149,720, issued Sep. 22, 1992), which is a continuation-in-part ofU.S. application Ser. No. 07/743,947, filed Aug. 12, 1991 (nowabandoned), and U.S. application Ser. No. 07/935,938, filed Aug. 27,1992 now U.S. Pat. No. 5,318,554, which is a continuation of U.S.application Ser. No. 07/743,950, filed Aug. 12, 1991 (now U.S. Pat. No.5,147,345, issued Sep. 15, 1992).

FIELD OF THE INVENTION

This application relates to flexible, microporous, open-celled polymericfoam materials having fluid absorption and retention characteristicsthat make them particularly suitable for absorbing aqueous body fluids,e.g., urine. This application particularly relates to absorbent foammaterials that remain relatively thin until wetted with such fluids.

BACKGROUND OF THE INVENTION

The development of highly absorbent articles for use as disposablediapers, adult incontinence pads and briefs, and catamenial productssuch as sanitary napkins, are the subject of substantial commercialinterest. A highly desired characteristic for such products is thinness.For example, thinner diapers are less bulky to wear, fit better underclothing, and are less noticeable. They are also more compact in thepackage, making the diapers easier for the consumer to carry and store.Compactness in packaging also results in reduced distribution costs forthe manufacturer and distributor, including less shelf space required inthe store per diaper unit.

The ability to provide thinner absorbent articles such as diapers hasbeen contingent on the ability to develop relatively thin absorbentcores or structures that can acquire and store large quantities ofdischarged body fluids, in particular urine. In this regard, the use ofcertain particulate absorbent polymers often referred to as "hydrogels,""superabsorbents" or "hydrocolloid" materials has been particularlyimportant. See, for example, U.S. Pat. No. 3,699,103 (Harper et al),issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun.20, 1972, that disclose the use of such particulate absorbent polymersin absorbent articles. Indeed, the development of thinner diapers hasbeen the direct consequence of thinner absorbent cores that takeadvantage of the ability of these particulate absorbent polymers toabsorb large quantities of discharged aqueous body fluids, typicallywhen used in combination with a fibrous matrix. See, for example, U.S.Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat.No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclosedual-layer core structures comprising a fibrous matrix and particulateabsorbent polymers useful in fashioning thin, compact, nonbulky diapers.

These particulate absorbent polymers are unsurpassed in their ability toretain large volumes of fluids, such as urine. A representative exampleof such particulate absorbent polymers are lightly crosslinkedpolyacrylates. Like many of the other absorbent polymers, these lightlycrosslinked polyacrylates comprise a multiplicity of anionic (charged)carboxy groups attached to the polymer backbone. It is these chargedcarboxy groups that enable the polymer to absorb aqueous body fluids asthe result of osmotic forces.

Besides osmotic forces, absorbency based on capillary forces is alsoimportant in many absorbent articles, including diapers. Capillaryforces are notable in various everyday phenomena, as exemplified by apaper towel soaking up spilled liquids. Capillary absorbents can offersuperior performance in terms of the rate of fluid acquisition andwicking, i.e. the ability to move aqueous fluid away from the point ofinitial contact. Indeed, the dual-layer core absorbent structures notedabove use the fibrous matrix as the primary capillary transport vehicleto move the initially acquired aqueous body fluid throughout theabsorbent core so that it can be absorbed and retained by theparticulate absorbent polymer positioned in layers or zones of the core.

An alternative absorbent material potentially capable of providingcapillary fluid transport would be open-celled polymeric foams. If madeappropriately, open-celled polymeric foams could provide features ofcapillary fluid acquisition, transport and storage required for use inhigh performance absorbent cores for absorbent articles such as diapers.Absorbent articles containing such foams could possess desirable wetintegrity, could provide suitable fit throughout the entire period thearticle is worn, and could avoid degradation in shape during use. Inaddition, absorbent articles containing such foam structures could beeasier to manufacture on a commercial scale. For example, absorbentdiaper cores could simply be stamped out of continuous foam sheets andcould be designed to have considerably greater integrity and uniformitythan air-laid fibrous absorbent cores containing particulate absorbentpolymers. Such foams could also be molded in any desired shape, or evenformed into integral, unitary diapers.

Literature and commercial practice is replete with descriptions ofvarious types of polymeric foams that can imbibe a variety of fluids fora variety of purposes. Indeed, employment of certain types of polymericfoam materials as elements of absorbent articles such as diapers andcatamenial products has previously been suggested. See, for example,U.S. Pat. No. 4,029,100 (Karami), issued Jun. 14, 1977, that discloses ashape-retaining diaper that can employ a foam element in the crotch areaof its absorbent pad assembly in order to provide high wet resiliency.Certain types of polymeric foam materials have also been suggested asuseful in absorbent articles for the purpose of actually imbibing,wicking and/or retaining aqueous body fluids. See, for example, U.S.Pat. No. 3,563,243 (Lindquist), issued Feb. 6, 1971 (absorbent pad fordiapers and the like where the primary absorbent is a hydrophilicpolyurethane foam sheet); U.S. Pat. No. 4,554,297 (Dabi), issued Nov.19, 1985 (body fluid absorbing cellular polymers that can be used indiapers or catamenial products); U.S. Pat. No. 4,740,528 (Garvey et al),issued Apr. 26, 1988 (absorbent composite structures such as diapers,feminine care products and the like that contain sponge absorbents madefrom certain types of super-wicking, crosslinked polyurethane foams).

Although various polymeric foam materials have been suggested for use inabsorbent articles, there is still a need for absorbent foam materialshaving optimized combinations of features and characteristics that wouldrender such foams especially useful in commercially marketed absorbentproducts such as diapers. In terms of desired absorbencycharacteristics, including capillary fluid transport capability, it hasbeen determined that optimized absorbent, open-celled polymeric foamsshould have the following characteristics:

(a) a relatively greater affinity for absorbing body fluids thanexhibited by other components in the absorbent article so that the foammaterial can drain (partition) fluids from these other components andkeep the fluids stored within the foam structure;

(b) relatively good wicking and fluid distribution characteristics inorder for the foam to transport the imbibed urine or other body fluidaway from the initial impingement zone and into the unused balance ofthe foam structure, thus allowing for subsequent gushes of fluid to beaccommodated; and

(c) a relatively high storage capacity with a relatively high fluidcapacity under load, i.e. under compressive forces.

As previously noted, a thinner absorbent core is usually a requirementfor making relatively thin absorbent articles, such as diapers. However,providing absorbent polymeric foam materials that remain relatively thinin form until wetted with aqueous body fluids is not straightforward.The absorbent foam material needs to remain relatively thin duringnormal storage and use prior to being wetted. This relatively thinpolymeric foam material must additionally have the needed absorbencycharacteristics described above if it is to be useful in highperformance absorbent cores. Making relatively thin polymeric foams thatare sufficiently soft and flexible for comfort of the wearer is also nota trivial task.

Accordingly, it would be desirable to be able to make an open-celledabsorbent polymeric foam material that: (1) has adequate or preferablysuperior absorbency characteristics, including capillary fluid transportcapability, so as to be desirable in high performance absorbent coresused in absorbent articles such as diapers, adult incontinence pads orbriefs, sanitary napkins and the like; (2) is relatively thin duringnormal storage and use until wetted with aqueous body fluids; (3) issufficiently flexible and soft so as to provide a high degree of comfortto the wearer of the absorbent article; and (4) can be manufactured on acommercial scale, at relatively reasonable or low cost.

DISCLOSURE OF THE INVENTION

The present invention relates to relatively thin, collapsed (i.e.unexpanded), polymeric foam materials that, upon contact with aqueousbody fluids, expand and absorb such fluids. These absorbent polymericfoam materials comprise a hydrophilic, flexible, nonionic polymeric foamstructure of interconnected open-cells that provides a specific surfacearea per foam volume of at least about 0.025 m² /cc. The foam structurehas incorporated therein at least about 0.1% by weight of atoxicologically acceptable, hygroscopic, hydrated salt. In its collapsedstate, the foam structure has an expansion pressure of about 30,000Pascals or less. In its expanded state, the foam structure has adensity, when saturated at 88° F. (31.1° C.) to its free absorbentcapacity with synthetic urine having a surface tension of 65±5 dynes/cm,of from about 10 to about 50% of its dry basis density in its collapsedstate.

It is believed that the ability of the polymeric foams of the presentinvention to remain in a collapsed, unexpanded state is due to thecapillary pressures developed within the collapsed foam structure thatat least equals the force exerted by the elastic recovery tendency(i.e., expansion pressure) of the compressed polymer. Surprisingly,these collapsed polymeric foam materials remain relatively thin duringnormal shipping, storage and use conditions, until ultimately wettedwith aqueous body fluids, at which point they expand. Because of theirexcellent absorbency characteristics, including capillary fluidtransport capability, these collapsed polymeric foam materials areextremely useful in high performance absorbent cores for absorbentarticles such as diapers, adult incontinence pads or briefs, sanitarynapkins, and the like. These collapsed polymeric foam materials are alsosufficiently flexible and soft so as to provide a high degree of comfortto the wearer of the absorbent article.

The present invention further relates to a process for consistentlyobtaining such relatively thin, collapsed polymeric foam materials bypolymerizing a specific type of water-in-oil emulsion having arelatively small amount of an oil phase and a relatively greater amountof a water phase, commonly known in the art as High Internal PhaseEmulsions or "HIPE." The oil phase of these HIPE emulsions comprisesfrom about 67 to about 98% by weight of a monomer component having: (a)from about 5 to about 40% by weight of a substantially water-insoluble,monofunctional glassy monomer; (b) from about 30 to about 80% by weightof a substantially water-insoluble, monofunctional rubbery comonomer;(c) from about 10 to about 40% by weight of a substantiallywater-insoluble polyfunctional crosslinking agent component. The oilphase further comprises from about 2 to about 33% by weight of anemulsifier component that is soluble in the oil phase and will provide astable emulsion for polymerization. The water or "internal" phase ofthese HIPE emulsions comprises an aqueous solution containing from about0.2 to about 20% by weight of a water-soluble electrolyte. The weightratio of the water phase to the oil phase in these HIPE emulsions rangesfrom about 12:1 to about 100:1. The polymerized foam is subsequentlydewatered (with or without prior washing/treatment steps) to provide thecollapsed foam material.

An important aspect of the process of the present invention is to carryout the emulsion formation and polymerization steps in a manner suchthat coalescence of the relatively small water droplets formed in theHIPE emulsion is reduced. This leads to a number average cell size inthe resulting polymeric foam material of about 50 microns or less. It isbelieved that this reduction in coalescence of the relatively smallwater droplets formed in the HIPE emulsion, and the resulting smallernumber average cell size in the polymeric foam material, is a keymechanism behind consistent formation of relatively thin, collapsedpolymeric foam materials according to the present invention, andespecially collapsed foam materials having good absorbency and fluidtransport characteristics. This reduction in coalescence can beconsistently achieved by the use of certain emulsifier systems, by theuse of lower temperatures during polymerization (curing), or both, asdescribed hereafter. Moreover, these relatively thin, collapsedabsorbent polymeric foam materials can be consistently manufacturedaccording to the process of the present invention on a potentiallycommercial scale, and at a potentially reasonable or low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 of the drawings is a photomicrograph (1500× magnification) of anedge view of a cut section of a representative absorbent polymeric foamaccording to the present invention in its collapsed state.

FIG. 2 of the drawings is a photomicrograph of a cut section of arepresentative absorbent polymeric foam according to the presentinvention in its expanded state.

FIGS. 3a through 3d of the drawings are photomicrographs (500×magnification) of cut sections of absorbent polymeric foams prepared bypolymerizing/curing a certain HIPE emulsion at different temperatures.

FIG. 4 of the drawings is a photomicrograph (1000× magnification) of acut section of an absorbent polymeric foam according to the presentinvention that is prepared from a HIPE emulsion containing a preferredco-emulsifier system.

FIG. 5 of the drawings is a cutaway depiction of a disposable diaperwhich utilizes the absorbent polymeric foam of the present invention asan hourglass-shaped fluid storage/distribution component in an absorbentdiaper core of dual-layer configuration.

FIG. 6 of the drawings represents a cut-away view of a form-fittingarticle such as a disposable training pants product which employs anabsorbent polymeric foam according to the present invention as anabsorbent core.

FIG. 7 of the drawings represents a blown-apart view of the componentsof a diaper structure also of dual layer core configuration having anhourglass-shaped fluid acquisition layer overlying an absorbent foamfluid storage/distribution layer with a modified hourglass shape.

DETAILED DESCRIPTION OF THE INVENTION

Polymeric foams of the type referred to herein can be characterized asthe structures which result when a relatively monomer-free liquid isdispersed as droplets or "bubbles" in a polymerizable monomer-containingliquid, followed by polymerization of the monomers in themonomer-containing liquid which surrounds the droplets. The resultingpolymerized dispersion can be in the form of a porous solidifiedstructure which is an aggregate of cells, the boundaries or walls ofwhich cells comprise solid polymerized material. The cells themselvescontain the relatively monomer-free liquid which, prior topolymerization, had formed the droplets in the liquid dispersion.

As described more fully hereafter, the collapsed polymeric foammaterials useful as absorbents in the present invention are typicallyprepared by polymerizing a particular type of water-in-oil emulsion.Such an emulsion is formed from a relatively small amount of apolymerizable monomer-containing oil phase and a relatively largeramount of a relatively monomer-free water phase. The relativelymonomer-free, discontinuous "internal" water phase thus forms thedispersed droplets surrounded by the continuous monomer-containing oilphase. Subsequent polymerization of the monomers in the continuous oilphase forms the cellular foam structure. The aqueous liquid remaining inthe foam structure after polymerization can be removed by pressing,thermal drying and/or vacuum dewatering.

Polymeric foams, including foams prepared from water-in-oil emulsions,can be relatively closed-celled or relatively open-celled in character,depending upon whether and/or the extent to which, the cell walls orboundaries, i.e., the cell windows, are filled with, or void of,polymeric material. The polymeric foam materials useful in the absorbentarticles and structures of the present invention are those which arerelatively open-celled in that the individual cells of the foam are forthe most part not completely isolated from each other by polymericmaterial of the cell walls. Thus the cells in such substantiallyopen-celled foam structures have intercellular openings or "windows"which are large enough to permit ready fluid transfer from one cell tothe other within the foam structure.

In substantially open-celled structures of the type useful herein, thefoam will generally have a reticulated character with the individualcells being defined by a plurality of mutually connected, threedimensionally branched webs. The strands of polymeric material whichmake up the branched webs of the open-cell foam structure can bereferred to as "struts." Open-celled foams having a typical strut-typestructure are shown by way of example in the photomicrograph set forthas FIG. 2. For purposes of the present invention, a foam material is"open-celled" if at least 80% of the cells in the foam structure thatare at least 1 micron size are in fluid communication with at least oneadjacent cell. Alternatively, a foam material can be considered to besubstantially open-celled if it has a measured available pore volumethat is at least 80% of the theoretically available pore volume, e.g.,as determined by the water-to-oil weight ratio of the HIPE emulsion fromwhich the foam material is formed.

In addition to being open-celled, the collapsed polymeric foam materialsof this invention are hydrophilic in character. The foams herein must besufficiently hydrophilic to permit the foam to absorb aqueous bodyfluids in the amounts hereafter specified. As discussed hereafter withrespect to preferred foam types and methods of foam preparation, theinternal surfaces of the foam structures herein can be renderedhydrophilic by virtue of residual hydrophilizing agents left in the foamstructure after polymerization or by virtue of selectedpost-polymerization foam treatment procedures which can be used to alterthe surface energy of the material which forms the foam structure.

The extent to which polymeric foam structures such as those of thisinvention are "hydrophilic" can be quantified by referencing the"adhesion tension" exhibited by such foams in contact with an absorbabletest liquid. Adhesion tension is defined by the formula

    AT=γ COS θ

wherein AT is adhesion tension in dynes/cm;

γ is the surface tension of a test liquid absorbed by the foam materialin dynes/cm;

θ is the contact angle in degrees between the surface of the polymericfoam and the vector which is tangent to the test liquid at the pointthat the test liquid contacts the foam surface.

For any given hydrophilic foam material, the adhesion tension exhibitedby the foam can be determined experimentally using a procedure wherebyweight uptake of a test liquid, e.g., synthetic urine, is measured for afoam sample of known dimensions and capillary suction specific surfacearea. Such a procedure is described in greater detail in the TESTMETHODS section hereafter. The foams which are useful as absorbents inthe present invention are generally those which have been renderedhydrophilic to the extent that they exhibit an adhesion tension of fromabout 15 to about 65 dynes/cm, more preferably from about 20 to about 65dynes/cm, as determined by capillary suction uptake of synthetic urinehaving a surface tension of 65±5 dynes/cm.

In addition to being "open celled" and "hydrophilic," the polymeric foammaterials useful in the present invention have a specific set ofstructural and mechanical properties, features and/or characteristics.It should be understood that these foam materials can have differentproperties, features and/or characteristics at different times prior tocontact between the foam and the aqueous body fluid to be absorbed. Forexample, during their manufacture, shipping, storage, etc., the foamsherein can have density and/or cell size values outside the ranges setforth hereafter for these parameters. However, such foams arenevertheless still within the scope of this invention if they laterundergo physical or rheological changes so that they have the requisitevalues specified hereafter for these properties, features and/orcharacteristics at at least some point prior to and/or during contactwith the aqueous body fluid to be absorbed.

The properties, features and/or characteristics of the polymeric foammaterials of the present invention are somewhat interrelated andinterdependent, but can be essentially categorized as follows: (I) thoseparticularly relevant to its collapsed state; (II) those particularlyrelevant to its expanded state; (III) those equally relevant to eitherits collapsed or expanded state; and (IV) those particularly relevant toits absorbency when in contact with aqueous body fluids.

I. Collapsed State

The collapsed polymeric foam materials of the present invention areusually obtained by polymerizing a HIPE-type emulsion as describedhereafter. These are water-in-oil emulsions having a relatively smallamount of an oil phase and a relatively greater amount of a water phase.Accordingly, after polymerization, the resulting foam contains asubstantial amount of water. This water can be expressed from the foamby compressive forces, and/or Can be reduced by thermal drying, such asby heating in an oven, or by vacuum dewatering. After compression,and/or thermal drying/vacuum dewatering, the polymeric foam material isin a collapsed, or unexpanded state.

The cellular structure of a representative collapsed HIPE foam fromwhich water has been expressed by compression is shown in thephotomicrograph set forth in FIG. 1. As shown in FIG. 1, the cellularstructure of the foam is distorted, especially when compared to the HIPEfoam structure shown in FIG. 2. (The foam structure shown in FIG. 2 isin its expanded state.) As can also be seen in FIG. 1, the voids orpores (dark areas) in the foam structure have been flattened orelongated. There is also no or minimal contact between adjacent strutsof the foam structure.

Following compression and/or thermal drying/vacuum dewatering, thecollapsed polymeric foam material can potentially re-expand, especiallywhen wetted with aqueous body fluids. (See FIG. 2 which shows a typicalHIPE foam structure according to the present invention in its expandedstate.) Surprisingly, polymeric foam materials of the present inventionremain in this collapsed, or unexpanded state, for significant periodsof time, e.g., up to at least about 1 year. The ability of the polymericfoam materials of the present invention to remain in thiscollapsed/unexpanded state is believed to be due to the capillaryforces, and in particular the capillary pressures developed within thefoam structure. As used herein, "capillary pressure" refers to thepressure differential across the liquid/air interface due to thecurvature of meniscus within the narrow confines of the pores in thefoam. [See Chatterjee, "Absorbency," Textile Science and Technology,Vol. 7, 1985, p, 36.] For wetting liquids, this is essentially apressure drop compared to atmospheric.

After compression, and/or thermal drying/vacuum dewatering, thepolymeric foam material of the present invention has residual water thatincludes both the water of hydration associated with the hydroscopic,hydrated salt incorporated therein (as described hereafter), as well asfree water absorbed within the foam. It is this residual water (assistedby the hydrated salts) that is believed to exert capillary pressures onthe resulting collapsed foam structure. Collapsed polymeric foammaterials of the present invention can have residual water contents ofat least about 4%, typically from about 4 to about 30%, by weight of thefoam when stored at ambient conditions of 72° F. (22° C.) and 50%relative humidity. Preferred collapsed polymeric foam materials of thepresent invention have residual water contents of from about 5 to about15% by weight of the foam.

In its collapsed state, the capillary pressures developed within thefoam structure at least equal the forces exerted by the elastic recoveryor modulus of the compressed polymer. In other words, the capillarypressure necessary to keep the collapsed foam material relatively thinis determined by the countervaling force exerted by the compressedpolymeric foam as it tries to "spring back." The elastic recoverytendency of polymeric foams can be determined from stress-strainexperiments where the expanded foam is compressed to about 25% of itsoriginal, expanded caliper (thickness) and then held in this compressedstate until an equilibrium or relaxed stress value is measured.Alternatively, and for the purposes of the present invention, theequilibrium relaxed stress value is determined from measurements on thepolymeric foam in its collapsed state when in contact with aqueousfluids, e.g., water. This alternative relaxed stress value is hereafterreferred to as the "expansion pressure" of the foam. A detaileddescription of a procedure for determining the expansion pressure offoams is set forth in the TEST METHODS section hereafter.

The expansion pressure for polymeric foams of the present invention isabout 30 kiloPascals (kPa) or less and typically from about 7 to about20 kPa, i.e. the expansion pressure is within a relatively narrow range.This means the elastic recovery tendency of typical polymeric foamsaccording to the present invention is relatively constant. Accordingly,the capillary pressures necessary to provide collapsed, unexpandedpolymeric foam materials according to the present invention alsotypically fall within a constant range.

It might be theoretically possible to measure directly the capillarypressures developed within the foam structure of collapsed polymericfoams according to the present invention. For example, if modeled simplyas a hollow cylinder, the capillary pressure (P) can be defined by theLaplace equation: ##EQU1## where γ is the surface tension of the fluid,θ is the contact angle, and r_(c) is the radius of a capillary tube.However, because of various complicating factors, including thedifficulty in assigning a value to r_(c) for the polymeric foam, thecapillary pressures developed within the foam structures of the presentinvention are not measured directly. Instead, the capillary pressuresdeveloped within the foam structure are more easily estimated byrewriting the above Laplace equation in the following, more generalform, that is applicable to any porous structure (e.g., foams): ##EQU2##wherein S_(c) is the capillary suction specific surface area of the foamstructure, ρ is the foam density, φ is the porosity of the foam, and γcos θ is the adhesion tension (AT) between the fluid and the foamstructure. Assuming the value for φ is close to 1 (typically the casewhen the foam structure is in its expanded state), the capillarypressures developed within the foam structure are essentially a functionof: (A) the capillary suction specific surface area; (B) the foamdensity; and (C) the adhesion tension between the fluid and the foamstructure.

For a constant adhesion tension value, it has been found that thespecific surface area per foam volume is particularly useful forempirically defining foam structures of the present invention that willremain in a collapsed state. As used herein, "specific surface area perfoam volume" refers to the capillary suction specific surface area ofthe foam structure times the foam density, i.e. the value S_(c) p in theabove general equation. This specific surface area per foam volume valueis characterized as "empirical" in that it is derived from (a) thecapillary suction specific surface area which is measured during wettingof the dried foam structure, and (b) the density of the expanded foamstructure after wetting to saturation, rather than by direct measurementof the dried, collapsed foam structure. Even so, it has been found thatcertain minimum specific surface area per foam volume values arecorrelatable to the ability of the foam structure to remain in acollapsed state. Polymeric foams according to the present inventionhaving specific surface area per foam volume values of at least about0.025 m² /cc, preferably at least about 0.05 m² /cc, most preferably atleast about 0.07 m² /cc have been found empirically to remain in acollapsed state.

Capillary suction specific surface area, foam density and the surfacetension component of adhesion tension of the fluid, as well as otherfactors relevant to the capillary pressure and/or expansion pressure ofthe polymeric foam, are discussed more fully hereafter:

A) Capillary Suction Specific Surface Area

Capillary suction specific surface area is, in general, a measure of thetest-liquid-accessible surface area of the polymeric network forming aparticular foam per unit mass of the bulk foam material (polymerstructural material plus solid residual material). Capillary suctionspecific surface area is determined both by the dimensions of thecellular units in the foam and by the density of the polymer. Capillarysuction specific surface area is thus a way of quantifying the totalamount of solid surface provided by the foam network to the extent thatsuch a surface participates in absorbency.

The capillary suction specific surface area of an open-celled foamstructure such as the polymeric foams herein is a key feature thatinfluences the capillarity (or capillary suction) exhibited by the foam.Foams of relatively high capillary suction specific surface area and lowdensity provide the very desirable combination of high capacity and highcapillarity. High specific surface area is also a consequence of thefineness of the struts making up the foam structure. It has been foundthat foam capillarity must be controlled and selected so that the foammaterials herein have sufficient capillarity to provide acceptable fluidretention and wicking rate of the fluid to occur within the foamstructure. Adjustment of capillary suction specific surface area, aswell as control of the hydrophilicity of the foam polymer surfaces, isthus the means for providing the requisite degree of capillarity for theabsorbent foams of this invention.

The capillary suction specific surface area is particularly relevant towhether adequate capillary pressures are developed within the foamstructure to keep it in a collapsed state until wetted with aqueous bodyfluids. Using the general form of the Laplace equation above, thecapillary pressure developed within the foam structure is proportionalto the capillary suction specific surface area. Assuming other factorssuch as the foam density and adhesion tension of the fluid are constant,this means that, as the capillary suction specific surface area isincreased (or decreased) the capillary pressure within the foamstructure also increases (or decreases) proportionately.

The capillary suction specific surface area of the foam herein can beinfluenced and controlled by adjusting various compositional andprocessing parameters that affect foam formation. For HIPEemulsion-based foams, compositional parameters include the water-to-oilratio of the HIPE emulsion, and the type and amounts of monomers,emulsifiers, and electrolytes utilized in the HIPE emulsion. Processparameters affecting capillary suction specific surface area includemixing energy and temperature.

For purposes of this invention, capillary suction specific surface areais determined by measuring the amount of capillary uptake of a lowsurface tension liquid (e.g., ethanol) which occurs within a foam sampleof a known mass and dimensions. A detailed description of such aprocedure for determining foam specific surface area via the capillarysuction method is set forth in the TEST METHODS section hereafter. Anyreasonable alternative method for determining capillary suction specificsurface area can also be utilized.

The collapsed open-celled, absorbent polymeric foams which are useful inthe present invention are those that have a capillary suction specificsurface area of at least about 0.3 m² /g. Typically, the capillarysuction specific surface area is in the range from about 0.7 to about 8m² /g, preferably from about 1 to about 7 m² /g, most preferably fromabout 1.5 to about 6 m² /g. For pore volumes to be defined hereafter,hydrophilic foams having such capillary suction specific surface areavalues will generally possess an especially desirable balance ofabsorbent capacity, fluid-retaining and fluid-wicking or distributioncharacteristics for aqueous body liquids such as urine. In addition,foams having such capillary suction specific surface areas can develop asufficient capillary pressure to keep the foam in a collapsed,unexpanded state until wetted with such aqueous body fluids.

B) Foam Density

Foam density in grams of foam per cubic centimeter of foam volume in airis specified herein on a dry basis. Thus the amount of absorbed aqueousliquid, e.g., residual salts and liquid left in the foam, for example,after HIPE emulsion polymerization, washing and/or hydrophilization, isdisregarded in calculating and expressing foam density. Foam density asspecified herein does include, however, other residual materials such asemulsifiers present in the polymerized foam. Such residual materialscan, in fact, contribute significant mass to the foam material.

The density of the foam materials herein, like capillary suctionspecific surface area, can influence a number of performance andmechanical characteristics of the absorbent foams herein. These includethe absorbent capacity for aqueous body fluids, the extent and rate offluid distribution within the foam and the foam flexibility andcompression deflection characteristics. Importantly also, the density ofthe foam absorbent structures herein can determine the costeffectiveness of such structures. Most importantly, foam density canpartially determine what capillary pressures are sufficient to keep thefoam in a collapsed, unexpanded state over substantial periods of timeuntil wetted with aqueous body fluids.

Any suitable gravimetric procedure which will provide a determination ofmass of solid foam material per unit volume of foam structure can beused to measure foam density. For example, an ASTM gravimetric proceduredescribed more fully in the TEST METHODS section hereafter is one methodwhich can be employed for density determination. For those situationswhere the foam sample preparation procedures (drying, aging, preflexing,etc.,) can inadvertently alter the density measurements obtained, thenalternate density determination tests can also be utilized. Suchalternative methods, for example, can include gravimetric densitymeasurements using a test liquid absorbed within the foam material. Thistype of density determination method can be useful for characterizingvery low density foams such as the foams herein wherein the dry densityapproximates the inverse of the pore volume of the foam. [SeeChatterjee, "Absorbency," Textile Science and Technology, Vol. 7, 1985,p. 41.] As with capillary suction specific surface area, the ranges forfoam density set forth hereafter are intended to be inclusive, i.e.,they are intended to encompass density values that can be determined byany reasonable experimental test method.

The collapsed absorbent polymeric foams of the present invention havedry basis density values in the range of from about 0.05 to about 0.4g/cm³, preferably from about 0.07 to about 0.25 g/cm³, and mostpreferably from about 0.1 to about 0.2 g/cm³. The density of the foammaterials can be adjusted to within the foregoing ranges by controlling,in particular, the water-to-oil ratio of the HIPE emulsion.

C) Surface Tension of Fluid

As previously noted, the collapsed polymeric foam materials of thepresent invention have a certain level of residual water. In the absenceof other surface tension modifying agents, pure water has a surfacetension of about 73 dynes/cm at 22° C. However, this residual watertypically contains other materials that will either increase or decreaseits surface tension. These materials can be present in the water phaseof the HIPE emulsion from which the polymeric foam materials of thepresent invention are typically made. These materials can also beincluded in the residual water as a result of post-polymerization steps,e.g., to hydrophilize the foam surface.

One such material present in the residual water of the collapsedpolymeric foam is a toxicologically acceptable hydroscopic, hydratablesalt, preferably calcium chloride. In addition to keeping the residualwater in the foam structure from evaporating, these hydratable salts canalso increase the surface tension of the water. For example, inclusionof 1% by weight calcium chloride increases the surface tension of waterto about 75 dynes/cm at 22° C. These hydratable salts are present in thefoam structure in an amount of at least about 0.1% by weight of thefoam, and typically in the range of from about 0.1 to about 8%,preferably from about 3 to about 6%, by weight of the foam.

Another material that is present in the collapsed polymeric foammaterials are certain oil-soluble emulsifiers. Representativeoil-soluble emulsifiers that can be present in the foam structures ofthe present invention include sorbitan laurate (e.g., SPAN® 20),mixtures of sorbitan laurate with sorbitan palmitate (e.g., SPAN® 40),mixtures of sorbitan laurate with certain polyglycerol fatty acid estersto be described hereafter, and sorbitan oleate (e.g., SPAN® 80). Theseemulsifiers are typically incorporated for the purpose of rendering thesurface of the foam structure hydrophilic. However, the morewater-soluble components in these emulsifiers can also be dissolved inthe residual water present in the foam structure and can affect itssurface tension, typically by decreasing it. These oil solubleemulsifiers are present in the foam structure in an amount of at leastabout 0.5% by weight of the foam and typically in the range of fromabout 0.5 to about 20%, preferably from about 5 to about 12%, by weightof the foam.

D) Cell Size

An alternative feature which can be useful in defining preferredcollapsed polymeric foam materials of this invention is cell size. Foamcells, and especially cells which are formed by polymerizing amonomer-containing oil phase that surrounds relatively monomer-freewater-phase droplets, will frequently be substantially spherical inshape. The size or "diameter" of such substantially spherical cells isthus yet another commonly utilized parameter for characterizing foams ingeneral as well as for characterizing certain preferred absorbent foamstructures of the present invention. Since cells in a given sample ofpolymeric foam will not necessarily be of approximately the same size,an average cell size, i.e., average cell diameter, will often bespecified.

As with foam density, and capillary suction specific surface area, cellsize is a foam parameter which can also impact on a number of importantmechanical and performance features of the absorbent foam material ofthis invention. Since cell size contributes to capillary suctionspecific surface area which, together with foam hydrophilicity,determine the capillarity of the foam, cell size is a foam structureparameter that can directly affect the internal fluid wicking propertiesof the foam absorbents herein, as well as the capillary pressure that isdeveloped within the foam structure.

A number of techniques are available for determining the average cellsize of foams. These techniques include mercury porosimetry methodswhich are well known in the art. The most useful technique, however, fordetermining cell size in foams involves a simple measurement based onthe scanning electron photomicrograph of a foam sample. FIG. 2, forexample, shows a typical HIPE foam structure according to the presentinvention in its expanded state. Superimposed on the photomicrograph isa scale representing a dimension of 20 microns. Such a scale can be usedto determine average cell size via an image analysis procedure. Imageanalysis of photomicrographs of foam samples is, in fact, a commonlyemployed analytical tool which can be used to determine average cellsize of the foam structures herein. Such a technique is described ingreater detail in U.S. Pat. No. 4,788,225 (Edwards et al), issued Nov.29, 1988, which is incorporated by reference.

The cell size measurements given herein are based on the number averagecell size of the foam in its expanded state e.g., as shown in FIG. 2.The foams useful as absorbents for aqueous body fluids in accordancewith the present invention will preferably have a number average cellsize of about 50 microns or less and typically in the range of fromabout 5 to about 50 microns. More preferably, the number average cellsize will be in the range from about 5 to about 40 microns, mostpreferably, from about 5 to about 35 microns.

The size or diameter of the cells in the foams herein can be influencedand controlled by variation of the same type of foam compositional andprocessing features that influence capillary suction specific surfacearea and foam density. For the preferred HIPE-based foams, these includeprimarily those factors which determine the size of the water-phasedroplets in the HIPE emulsion precursor of the polymeric foam structuresherein. Thus, cell size can be varied by adjusting the energy inputduring mixing and the type and amount of emulsifier used to form theHIPE emulsion.

E) Plasticization Effects

The oil-soluble emulsifiers present in the collapsed foam structure canaffect the polymer modulus itself, e.g., as a plasticizer. Theseplasticizer effects can either be the result of the incorporation of theemulsifier during the formation of the HIPE emulsion, or else can be dueto incorporation as a result of post-polymerization hydrophilizationtreatments. Plasticization by the emulsifiers usually tends to decreasethe polymer modulus and thus lowers the expansion pressure of the foamstructure. This means that the capillary pressure required for stablecollapsed foam structures can be somewhat reduced. However,plasticization to too great an extent can also be undesirable. Forexample, the polymeric foam structure can be plasticized to the extentthat the foam is weakened and does not have the resistance tocompression deflection characteristics specified hereafter.

II. Expanded State

A) Density Upon Saturation With Synthetic Urine

A particularly important property of the absorbent foams of the presentinvention in their expanded state is their density upon saturation withaqueous body fluids, relative to the dry basis density of the absorbentfoam in its collapsed state. The density of the expanded foam whensaturated with aqueous body fluids, relative to its dry basis density inits collapsed (compressed) state, provides a measure of the relativethickness of the foam in its expanded state. This provides aparticularly relevant measure of how thin the foam is when expanded andwhen saturated with aqueous body fluids.

For the purposes of the present invention, the density of the absorbentfoams in their expanded state is measured when it is saturated at 88° F.(31.1° C.) to its free absorbent capacity with synthetic urine having asurface tension of 65±5 dynes/cm. The density of the foam in itsexpanded state when saturated with the synthetic urine can be measuredby a procedure described more fully hereafter in the Test Methodssection. The density of the foam measured in its expanded, saturatedstate, is then related, as a percentage, to the dry basis density of thefoam in its collapsed state. For the purposes of the present invention,the density of the foam in its expanded state upon saturation to itsfree absorbent capacity with synthetic urine can be in the range of fromabout 10 to about 50% of its dry basis density in its collapsed state,and is preferably in the range of from about 10 to about 30%, mostpreferably from about 15 to about 25%.

B) Pore Volume

Pore volume is a measure of the volume of the openings or cells in aporous foam structure per unit mass of solid material (polymer structureplus any residual solids) which forms the foam structure. Pore volumecan be important in influencing a number of performance and mechanicalfeatures of the absorbent foams herein, especially in their expandedstate. Such performance and mechanical features include absorbentcapacity of the foams for aqueous body fluids, foam flexibility and foamcompression deflection characteristics.

Pore volume can be determined by any suitable experimental method whichwill give an accurate indication of the actual pore volume of thestructure. Such experimental methods will generally involve themeasurement of the volume and/or mass of a test liquid which can beintroduced into the foam structure and which therefore is representativeof the volume occupied by the open cells of the foam. For this reasonthe pore volume parameter of the foams herein can also be referred to as"available pore volume."

One conventional way for determining available pore volumeexperimentally involves the introduction of a low surface tension,nonpolymer-swelling liquid such as 2-propanol into the foam structure. Aprocedure for determining available pore volume using 2-propanol is setforth hereafter in the TEST METHODS section. It should be understood,however, that alternative test liquids and procedures can also be usedto determine available pore volume.

The pore volume of the absorbent foams useful herein can be influencedand controlled by adjusting a number of foam compositional andprocessing features. For example, with the preferred HIPE emulsion-basedfoams herein, these pore volume influencing features can include thewater-to-oil ratio of the HIPE emulsion, type and amount of water phaseelectrolyte used, type and amount of oil phase emulsifier used,post-polymerization steps to effect washing and/or densification of thefoam and degree of recovery of the polymerized foam structure after suchsteps.

The foam materials of the present invention will generally have a porevolume of from about 12 to about 100 mL/g, more preferably from about 20to about 70 mL/g, and most preferably from about 25 to about 50 mL/g.Such ranges for pore volume are intended to be an "inclusive" definitionof theoretical pore volume for the foams encompassed by this invention.Thus if any experimental method which can reasonably be expected to givemeasurements approximating theoretical pore volume provides valueswithin the foregoing ranges, then the foam materials tested by any suchmethod are within the scope of this invention.

C) Resistance to Compression Deflection

An important mechanical feature of the polymeric foams of this inventionis the strength of the absorbent foam, in its expanded state, asdetermined by its resistance to compression deflection. The resistanceto compression deflection exhibited by the foams herein is a function ofthe polymer elastic modulus and the density of the foam network. Thepolymeric elastic modulus is, in turn, determined by a) the polymericcomposition; b) the extent to which the polymeric foam is plasticized byresidual material, e.g., emulsifiers, left in the foam structure afterprocessing; and c) the conditions under which the foam was polymerized.

To be useful as absorbent structures in absorbent articles such asdiapers, the absorbent foam materials of the present invention must besuitably resistant to deformation or compression by forces encounteredwhen such absorbent materials are engaged in the absorption andretention of fluids. Foams which do not possess sufficient foam strengthin terms of resistance to compression deflection may be able to acquireand store acceptable amounts of body fluid under no-load conditions butwill too easily give up such fluid under the compressive stress causedby the motion and activity of the wearer of the absorbent articles whichcontain the foam.

The resistance to compression deflection exhibited by the polymericfoams of the present invention can be quantified by determining theamount of strain produced in a sample of saturated foam material heldunder a certain confining pressure for a specified period of time. Forthe purposes of the present invention such measurements can be made on afoam sample of standard size (cylinders which are 0.25 cm thick and havea cross-sectional circular area of 6.5 cm²). Such samples are saturatedwith synthetic urine having a surface tension of 65±5 dynes/cm and arethereafter subjected to a confining pressure of 5.1 kPa for a period of15 minutes at a temperature of 88° F. (31.1° C.). The amount of strainproduced in such testing is reported as a percentage of the saturatedand fully expanded sample thickness that the compressed thickness of thesample represents. The method for carrying out this particular type oftest for quantifying resistance to compression deflection is set forthhereafter in greater detail in the TEST METHODS section.

The absorbent foams useful herein are those which exhibit a resistanceto compression deflection such that a confining pressure of 5.1 kPaproduces a strain of typically from about 2 to about 80% compression ofthe foam structure when it has been saturated to its free absorbentcapacity with synthetic urine having a surface tension of 65±5 dynes/cm.Preferably the strain produced under such conditions will be in therange from about 5 to about 40%, most preferably from about 5 to about25%. For the preferred HIPE foams of this invention, resistance tocompression deflection can be adjusted to strain values within theforegoing ranges by appropriate selection of monomer, comonomer andcrosslinker types and concentrations in combination with the selectionof appropriate emulsion formation and emulsion polymerization conditionsand techniques. Thus, such preferred foams can be formed from materialswith elastic modulii large enough to provide adequate resistance tocompression deflection even though such foams are low density and havevery fine struts.

D) Recovery From Compression Deflection

Recovery from compression deflection relates to the tendency orpropensity of a piece of foam material to return to its originaldimensions after being deformed or compressed under forces encounteredin manufacture, storage or use. For purposes of the present invention,recovery from compression deflection of the preferred absorbent foamsherein are determined on foams which are in their expanded state, andcontain absorbed body fluid. Accordingly, recovery from compressiondeflection is measured on expanded foams which are saturated withsynthetic urine.

A suitable procedure for determining recovery from compressiondeflection is set forth in the TEST METHODS section hereafter. Such aprocedure in general involves compression and release of a standard sizefoam sample which has been saturated to its free absorbent capacity withsynthetic urine. Samples are maintained under 50% compression for a setperiod of time and then are released from compression. The extent towhich the sample recovers its thickness, in the presence of availablefree fluid, in the one-minute period after the release of compressiveforce is taken as a measure of the recovery from compression deflection(resilience) propensity of the sample.

Preferred absorbent foams of the present invention will generallyexhibit a recovery of at least 75% of the expanded caliper when wetafter one minute. More preferably, such preferred foam materials willhave a recovery from compression deflection at least 80% when wet.

III. Collapsed or Expanded State

A) Flexibility

The absorbent foams of the present invention should be sufficientlyflexible so that they can be utilized in absorbent products that willconform to the body shape of the wearer. Characterization of theabsorbent foams herein as flexible, therefore, means that these foamscan be deformed or bent to the extent necessary for use in suchabsorbent articles without significant damage to their structuralintegrity or significant loss of their absorbent properties.

Preferred absorbent foams of the present invention should also besufficiently flexible to withstand compressive or deforming forces whichare encountered during preparation, processing, packaging, shipping andstoring of absorbent articles containing such foam materials. Disposablediapers, for example, are generally packaged and marketed in a foldedcondition wherein the diaper core is folded in both the longitudinal andtransverse directions. Disposable diapers are also generally marketed inthe form of stacks of folded diapers, which stacks are contained andcompressed by their surrounding packaging. Accordingly, the compressiveand deforming forces to which the foam absorbents herein can besubjected during processing and marketing can be even greater than thosewhich are applied to the foam materials in use.

Given the nature of treatment which the absorbent foams herein canencounter, preferred absorbent foam materials of this invention willpossess flexibility characteristics which can be quantified byreferencing their ability to withstand bending without undergoingsignificant damage to their structural integrity. Described in the TESTMETHODS section hereafter is a procedure for measuring the flexibilityof the absorbent foams herein by determining whether and how many timesa foam sample of a given specified size can be bent around a cylindricalmandrel at a specified rate without breaking. The preferred foams ofthis invention are those which are flexible enough so that, at theirpoint of use as an absorbent for body fluids, the saturated foammaterial at 88° F. (31.1° C.) can be subjected to this bending testwithout breaking, i.e., exhibit a bending value of at least one cycle.More preferably, preferred foams can be bent at least 3 times withoutbreaking when subjected to such a test procedure.

B) Foam Integrity and Softness

While not absolutely essential for the realization of operable or usefulabsorbent structures, the absorbent foams of this invention willpreferably possess the additional mechanical attributes of structuralintegrity in use and softness (lack of irritation) to the touch. Forexample, foam materials that will be employed in absorbent articles suchas infant diapers will frequently be subjected to both dynamic andstatic forces which arise when the wearer walks, runs, crawls or jumps.Such forces not only tend to compress the absorbent foams and expelfluid therefrom, but such forces also tend to rip or tear or otherwisefragment the foam structure. Obviously, it would be advantageous forfoam structures which are to be used in this manner to have sufficientstructural integrity to minimize the incidence of foam tearing orfragmenting in use.

The absorbent foams of this invention can also be used in absorbentarticles, as described more fully hereafter, in configurations whereinthe foam material surface can come in close proximity to or even inactual contact with the wearer's skin. Accordingly, it would be verydesirable for the surface of the absorbent foams herein to be acceptablysoft and non-irritating to the touch.

IV. Fluid Handling and Absorbency Characteristics

Absorbent foams having suitable polymeric compositions, and thestructural characteristics and mechanical features as hereinbeforedescribed, will in general exhibit especially desirable and useful bodyfluid handling and absorbency characteristics. Such fluid handling andabsorbency characteristics are in turn the attributes of the preferredfoam materials herein which render such foams especially suitable foruse as absorbent structures in absorbent articles designed to acquireand hold aqueous body fluids.

The fluid handling and absorbency characteristics which are mostrelevant to the realization of suitable absorbent foams are: A) the freeabsorbent capacity of the foam; B) the rate of vertical wicking of fluidthrough the foam structure; C) the absorbent capacity of the foam atspecific reference wicking heights; and D) the ability of the absorbentfoam structures to drain (partition) fluid from competing absorbentstructures with which the foam can be in contact. Each of thesecharacteristics is described in greater detail as follows:

A) Free Absorbent Capacity

Free absorbent capacity is the total amount of test fluid (syntheticurine) which a given foam sample will absorb into its cellular structureper unit mass of solid material in the sample. Such free absorbentcapacity measurements are for purposes herein calculated at equilibrium,i.e., after the foam sample has been allowed to acquire and/or hold allof the fluid it can over whatever time period is needed to form acompletely saturated foam sample with the test liquid. The foammaterials which are especially useful as absorbent structures inabsorbent articles such as diapers will at least meet a minimum freeabsorbent capacity.

Using the procedure described in greater detail hereafter in the TESTMETHODS section, free absorbent capacity can both be determined for anygiven foam sample by a gravimetric analysis technique. In such atechnique, a foam sample of specified known size and weight is placed ina dish of test fluid (synthetic urine) and is allowed to absorb the testfluid to equilibrium. After removal of the saturated sample from thefluid, the amount of fluid held per gram of foam, i.e., the measuredfree capacity, is then calculated. To be especially useful in absorbentarticles for absorbing urine, the absorbent foams of the presentinvention should have a free capacity of at least about 12, andpreferably at least about 20, mL of synthetic urine per gram of dry foammaterial.

B) Vertical Wicking Performance

Yet another fluid handling attribute of the absorbent foams usefulherein relates to their ability to quickly move or "wick" acceptableamounts of body fluids through their foam structures. Vertical wicking,i.e., fluid wicking in a direction opposite from gravitational force, isan especially desirable performance attribute for the absorbent foammaterials herein. This is because such materials will frequently beutilized in absorbent articles in a manner that fluid to be absorbedmust be moved within the article from a relatively lower position to arelatively higher position within the absorbent core of the article.

Vertical wicking performance is related to the magnitude of thecapillary suction driving force which moves liquid through the foam andholds it in the foam structure. Foam characterizing parameters whichrelate to vertical wicking propensity thus provide an indication as tohow well preferred foams herein will perform as absorbent structures inabsorbent articles. For the foam absorbents of the present invention,fluid wicking propensity can be quantified by referencing both avertical wicking rate test and a vertical wicking absorbent capacitytest.

1) Vertical Wicking Rate

The vertical wicking rate test measures the time taken for a coloredtest liquid (e.g., synthetic urine) from a reservoir to wick a verticaldistance of 5 cm through a test strip of foam of specified size when thetest is performed at 37° C. Such a vertical wicking rate test isdescribed in greater detail hereafter in the TEST METHODS section. To beespecially useful in absorbent articles for absorbing urine, the foamabsorbents of the present invention will preferably have a 5 cm verticalwicking rate of no more than about 30 minutes when wicking syntheticurine (65±5 dynes/cm). More preferably, the preferred foam absorbents ofthe present invention will have a 5 cm vertical wicking rate of no morethan about 5 minutes when wicking synthetic urine.

2) Vertical Wicking Absorbent Capacity

The vertical wicking absorbent capacity test is carried out inconjunction with the vertical wicking rate test. Vertical wickingabsorbent capacity measures the amount of test fluid per gram ofabsorbent foam that is wicked to each one inch (2.54 cm) verticalsection of the same standard size foam sample used in the verticalwicking rate test. Such a determination is generally made after thesample has been allowed to vertically wick test fluid to equilibrium(e.g, after about 18 hours). Like the vertical wicking rate test, thevertical wicking absorbent capacity test is described in greater detailhereafter in the TEST METHODS section.

To be especially useful in absorbent articles for absorbing urine, thepreferred absorbent foams of the present invention will generally have avertical wicking absorbent capacity such that, at 11.4 cm (4.5 inches)of vertical wicking height, the foam test strip wicks to at least about50%, most preferably at about 75%, of its free absorbent capacity.

C) Partitioning

The absorbent foam structures herein will frequently be utilized inabsorbent articles along with other types of absorbent structures whichcan also participate in acquiring, distributing and/or storingdischarged body fluids. In those contexts wherein the foam structuresherein are to serve primarily as a fluid storage/redistributioncomponent in absorbent articles, it is desirable for such foams to havea propensity for pulling body fluids into the foam structure from otherabsorbent components which also are absorbing such fluids. Such apropensity to drain fluid from other absorbent article components isknown in the art as "partitioning." The concept of partitioning andcertain procedures for determining partitioning performance aredescribed, for example, in U.S. Pat. No. 4,610,678 (Weisman et al),issued Sep. 9, 1986. When tested for partitioning performance usingprocedures similar to those disclosed in U.S. Pat. No. 4,610,678, theabsorbent foam structures of this invention exhibit especially desirablefluid partitioning characteristics.

V. Preparation of Collapsed Polymeric Foam Materials

As previously noted, collapsed polymeric foam materials according to thepresent invention can be prepared by polymerization of certainwater-in-oil emulsions having a relatively high ratio of water phase tooil phase. Emulsions of this type which have these relatively high waterto oil phase ratios are commonly known in the art as high internal phaseemulsions ("HIPEs" or "HIPE" emulsions). The polymeric foam materialswhich result from the polymerization of such emulsions are referred toherein as "HIPE foams."

The chemical nature, makeup and morphology of the polymer material whichforms the HIPE foam structures herein is determined by both the type andconcentration of the monomers, comonomers and crosslinkers utilized inthe HIPE emulsion and by the emulsion formation and polymerizationconditions employed. No matter what the particular monomeric makeup,molecular weight or morphology of the polymeric material might be, theresulting polymeric foams will generally be viscoelastic in character,i.e. the foam structures will possess both viscous, i.e. fluid-like,properties and elastic, i.e. spring-like, properties. It is alsoimportant that the polymeric material which forms the cellular foamstructure have physical, rheological, and morphological attributeswhich, under conditions of use, impart suitable flexibility, resistanceto compression deflection, and dimensional stability to the absorbentfoam material.

The relative amounts of the water and oil phases used to form the HIPEemulsions are, among many other parameters, important in determining thestructural, mechanical and performance properties of the resultingpolymeric foams. In particular, the ratio of water to oil in thefoam-forming emulsion can influence the foam density, cell size, andcapillary suction specific surface area of the foam and dimensions ofthe struts which form the foam. The emulsions used to prepare the HIPEfoams of this invention will generally have water-to-oil phase ratiosranging from about 12:1 to about 100:1, more preferably from about 20:1to about 70:1, most preferably from about 25:1 to about 50:1.

A. Oil Phase Components

The continuous oil phase of the HIPE emulsion comprises monomers thatare polymerized to form the solid foam structure. This monomer componentincludes a "glassy" monomer, a "rubbery" comonomer and a cross-linkingagent. Selection of particular types and amounts of monofunctionalmonomer(s) and comonomer(s) and polyfunctional cross-linking agent(s)can be important to the realization of absorbent HIPE foams having thedesired combination of structure, mechanical, and fluid handlingproperties which render such materials suitable for use in the inventionherein.

The monomer component utilized in the oil phase of the HIPE emulsionscomprises one or more monofunctional monomers that tend to impartglass-like properties to the resulting polymeric foam structure. Suchmonomers are referred to as "glassy" monomers, and are, for purposes ofthis invention, defined as monomeric materials which would produce highmolecular weight (greater than 6000) homopolymers having a glasstransition temperature, T_(g), above about 40° C. These monofunctionalglassy monomer types include methacrylate-based monomers (e.g., methylmethacrylate) and styrene-based monomers (e.g., styrene). The preferredmonofunctional glassy monomer type is a styrene-based monomer withstyrene itself being the most preferred monomer of this kind.Substituted, e.g., monosubstituted, styrene such as p-methylstyrene canalso be employed. The monofunctional glassy monomer will normallycomprise from about 5 to about 40%, more preferably from about 10 toabout 30%, more preferably from about 15 to about 25%, most preferablyabout 20%, by weight of the monomer component.

The monomer component also comprises one or more monofunctionalcomonomers which tend to impart rubber-like properties to the resultingpolymeric foam structure. Such comonomers are referred to as "rubbery"comonomers and are, for purposes of this invention, defined as monomericmaterials which would produce high molecular weight (greater than10,000) homopolymers having a glass transition temperature, T_(g), ofabout 40° C. or lower. Monofunctional rubbery comonomers of this typeinclude, for example, the C₄ -C₁₂ alkylacrylates, the C₆ -C₁₄alkylmethacrylates, and combinations of such comonomers. Of thesecomonomers, n-butylacrylate and 2-ethylhexylacrylate are the mostpreferred. The monofunctional rubbery comonomer will generally comprisefrom about 30 to about 80%, more preferably from about 50 to about 70%,most preferably from about 55 to about 65%, by weight of the monomercomponent.

Since the polymer chains formed from the glassy monomer(s) and therubbery comonomer(s) are to be cross-linked, the monomer component alsocontains a polyfunctional cross-linking agent. As with themonofunctional monomers and comonomers, selection of a particular typeand amount of cross-linking agent is very important to the eventualrealization of preferred polymeric foams having the desired combinationof structural, mechanical, and fluid-handling properties.

Depending upon the type and amounts of monofunctional monomers andcomonomers utilized, and depending further upon the desiredcharacteristics of the resulting polymeric foams, the polyfunctionalcross-linking agent can be selected from a wide variety ofpolyfunctional, preferably difunctional, monomers. Thus, thecross-linking agent can be a divinyl aromatic material such asdivinylbenzene, divinyltolulene or diallylphthalate. Alternatively,divinyl aliphatic crosslinkers such as any of the diacrylic ordimethylacrylic acid esters of polyols, such as 1,6-hexanediol and itshomologues, can be utilized. The cross-linking agent found to besuitable for preparing the preferred HIPE emulsions herein isdivinylbenzene. The cross-linking agent of whatever type will generallybe employed in the oil phase of the foam-forming emulsions herein in anamount of from about 10 to about 40%, more preferably from about 15 toabout 25%, most preferably about 20%, by weight of the monomercomponent.

The major portion of the oil phase of the HIPE emulsions will comprisethe aforementioned monomers, comonomers and crosslinking agents. It isessential that these monomers, comonomers and cross-linking agents besubstantially water-insoluble so that they are primarily soluble in theoil phase and not the water phase. Use of such substantiallywater-insoluble monomers insures that HIPE emulsions of appropriatecharacteristics and stability will be realized.

It is, of course, highly preferred that the monomers, comonomers andcross-linking agents used herein be of the type such that the resultingpolymeric foam is suitably non-toxic and appropriately chemicallystable. These monomers, comonomers and cross-linking agents shouldpreferably have little or no toxicity if present at very low residualconcentrations during post-polymerization foam processing and/or use.

Another essential component of the oil phase is an emulsifier whichpermits the formation of stable HIPE emulsions. Such emulsifiers arethose which are soluble in the oil phase used to form the emulsion.Emulsifiers utilized are typically nonionic and include the sorbitanfatty acid esters, the polyglycerol fatty acid esters, and combinationsthereof. Preferred emulsifiers include sorbitan laurate (e.g., SPAN®20), sorbitan oleate (e.g., SPAN® 80), combinations of sorbitan laurateand sorbitan palmitate (e.g., SPAN® 40) in a weight ratio of from about1:1 to about 3:1, and especially combinations of sorbitan laurate withcertain polyglycerol fatty acid esters to be described hereafter.

The oil phase used to form the HIPE emulsions will generally comprisefrom about 67 to about 98% by weight monomer component and from about 2to about 33% by weight emulsifier component. Preferably, the oil phasewill comprise from about 80 to about 95% by weight monomer component andfrom about 5 to about 20% by weight emulsifier component.

In addition to the monomer and emulsifier components, the oil phase cancontain other optional components. One such optional oil phase componentis an oil soluble polymerization initiator of the general type hereafterdescribed. Another possible optional component of the oil phase is asubstantially water insoluble solvent for the monomer and emulsifiercomponents. A solvent of this type must, of course, not be capable ofdissolving the resulting polymeric foam. Use of such a solvent is notpreferred, but if such a solvent is employed, it will generally compriseno more than about 10% by weight of the oil phase.

B. Water Phase Components

The discontinuous internal phase of the HIPE emulsions is the waterphase which will generally be an aqueous solution containing one or moredissolved components. One essential dissolved component of the waterphase is a water-soluble electrolyte. The dissolved electrolyte in thewater phase of the HIPE emulsion serves to minimize the tendency ofmonomers and crosslinkers which are primarily oil soluble to alsodissolve in the water phase. This, in turn, is believed to minimize theextent to which, during polymerization of the emulsion, polymericmaterial fills the cell windows at the oil/water interfaces formed bythe water phase droplets. Thus, the presence of electrolyte and theresulting ionic strength of the water phase is believed to determinewhether and to what degree the resulting preferred polymeric foams canbe open-celled.

Any electrolyte which provides ionic species to impart ionic strength tothe water phase can be used. Preferred electrolytes are mono-, di-, ortri-valent inorganic salts such as the water-soluble halides, e.g.,chlorides, nitrates and sulfates of alkali metals and alkaline earthmetals. Examples include sodium chloride, calcium chloride, sodiumsulfate and magnesium sulfate. Calcium chloride is the most preferredfor use in the present invention. Generally the electrolyte will beutilized in the water phase of the HIPE emulsions in a concentration inthe range of from about 0.2 to about 20% by weight of the water phase.More preferably, the electrolyte will comprise from about 1 to about 10%by weight of the water phase.

The HIPE emulsions will also typically contain a polymerizationinitiator. Such an initiator component is generally added to the waterphase of the HIPE emulsions and can be any conventional water-solublefree radical initiator. Materials of this type include peroxygencompounds such as sodium, potassium and ammonium persulfates, hydrogenperoxide, sodium peracetate, sodium percarbonate and the like.Conventional redox initiator systems can also be utilized. Such systemsare formed by combining the foregoing peroxygen compounds with reducingagents such as sodium bisulfite, L-ascorbic acid or ferrous salts.

The initiator material can comprise up to about 5 mole percent based onthe total moles of polymerizable monomers present in the oil phase. Morepreferably, the initiator comprises from about 0.001 to 0.5 mole percentbased on the total moles of polymerizable monomers in the oil phase.When used in the water-phase, such initiator concentrations can berealized by adding initiator to the water phase to the extent of fromabout 0.02% to about 0.4%, more preferably from about 0.1% to about0.2%, by weight of the water phase.

C. Hydrophilizing Agents and Hydratable Salts

The cross-linked polymer material that forms the collapsed absorbentfoam structures herein will preferably be substantially free of polarfunctional groups on its polymeric structure. Thus, immediately afterthe polymerization step, the polymer material which forms the foamstructure surfaces of such preferred absorbent foams will normally berelatively hydrophobic in character. Accordingly, preferredjust-polymerized foams can need further treatment to render the foamstructure surfaces relatively more hydrophilic so that such foams can beused as absorbents for aqueous body fluids. Hydrophilization of the foamsurfaces, if necessary, can generally be accomplished by treating thepolymerized HIPE foam structures with a hydrophilizing agent in a mannerdescribed more fully hereafter.

Hydrophilizing agents are any materials which will enhance the waterwettability of the polymeric surfaces with which they are contacted andonto which they are deposited. Hydrophilizing agents are well known inthe art, and can include surfactant materials, preferably of thenonionic type. Hydrophilizing agents will generally be employed inliquid form, and can be dissolved or dispersed in a hydrophilizingsolution which is applied to the HIPE foam surfaces. In this manner,hydrophilizing agents can be adsorbed onto the polymeric surfaces of thepreferred HIPE foam structures in amounts suitable for rendering suchsurfaces substantially hydrophilic but without altering the desiredflexibility and compression deflection characteristics of the foam. Inpreferred foams which have been treated with hydrophilizing agents, thehydrophilizing agent is incorporated into the foam structure such thatresidual amounts of the agent which remain in the foam structure are inthe range from about 0.5% to about 20%, preferably from about 5 to about12%, by weight of the foam.

One type of suitable hydrophilizing agent is a non-irritatingoil-soluble surfactant. Such surfactants can include all of thosepreviously described for use as the emulsifier for the oil phase of theHIPE emulsion, such as sorbitan laurate (e.g., SPAN® 20), andcombinations of sorbitan laurate with certain polyglycerol fatty acidesters to be described hereafter. Such hydrophilizing surfactants can beincorporated into the foam during HIPE emulsion formation andpolymerization or can be incorporated by treatment of the polymeric foamwith a solution or suspension of the surfactant dissolved or dispersedin a suitable carrier or solvent.

Another material that needs to be incorporated into the HIPE foamstructure is a hydratable, and preferably hygroscopic or deliquesent,water soluble inorganic salt. Such salts include, for example,toxicologically acceptable alkaline earth metal salts. Materials of thistype and their use in conjunction with oil-soluble surfactants as thefoam hydrophilizing agent is described in greater detail in the U.S.patent application Ser. No. 07/743,951, filed Aug. 12, 1991, thedisclosure of which is incorporated by reference. Preferred salts ofthis type include the calcium halides such as calcium chloride which, aspreviously noted, can also be employed as the electrolyte in the waterphase of the HIPE emulsions used to prepare the polymeric foams.

Hydratable inorganic salts can easily be incorporated into the polymericfoams herein by treating the foams with aqueous solutions of such salts.Solutions of hydratable inorganic salts can generally be used to treatthe foams after completion of, or as part of, the process of removingthe residual water phase from the just-polymerized foams. Contact offoams with such solutions is preferably used to deposit hydratableinorganic salts such as calcium chloride in residual amounts of at leastabout 0.1% by weight of the foam, and typically in the range of fromabout 0.1 to about 8%, preferably from about 3 to about 6%, by weight ofthe foam.

Treatment of preferred foam structures which are relatively hydrophobicas polymerized with hydrophilizing agents (with or without hydratablesalts) will typically be carried out to the extent that is necessary andsufficient to impart suitable hydrophilicity to the preferred HIPE foamsof the present invention. Some foams of the preferred HIPE emulsiontype, however, can be suitably hydrophilic as prepared and can haveincorporated therein sufficient amounts of hydratable salts, thusrequiring no additional treatment with hydrophilizing agents orhydratable salts. In particular, such preferred HIPE foams can be thosewherein sorbitan fatty acid esters such as sorbitan laurate (e.g., SPAN20), or combinations of sorbitan laurate with certain polyglycerol fattyacid esters to be described hereafter, are used as emulsifiers added tothe oil phase and calcium chloride is used as an electrolyte in thewater phase of the HIPE emulsion. In that instance, theresidual-emulsifier-containing internal polymerized foam surfaces willbe suitably hydrophilic, and the residual water-phase liquid willcontain or deposit sufficient amounts of calcium chloride, even afterthe polymeric foams have been dewatered.

D. Processing Conditions for Obtaining HIPE Foams

Foam preparation typically involves the steps of: 1) forming a stablehigh internal phase emulsion (HIPE); 2) polymerizing/curing this stableemulsion under conditions suitable for forming a solid polymeric foamstructure; 3) washing the solid polymeric foam structure to remove theoriginal residual water phase from the polymeric foam structure and, ifnecessary, treating the polymeric foam structure with a hydrophilizingagent and/or hydratable salt to deposit any needed hydrophilizingagent/hydratable salt, and 4) thereafter dewatering this polymeric foamstructure (preferably including compression in the z-direction) to theextent necessary to provide a collapsed, unexpanded polymeric foammaterial useful as an absorbent for aqueous body fluids.

To consistently obtain relatively thin, collapsed polymeric foammaterials according to the present invention, it has been found to beparticularly important to carry out the emulsion formation andpolymerization steps in a manner such that coalescence of the waterdroplets in the HIPE emulsion is reduced or minimized. HIPE emulsionsare not always stable, particularly when subjected to higher temperatureconditions to effect polymerization and curing. As the HIPE emulsiondestabilizes, the water droplets present in it can aggregate together,and coalesce to form much large water droplets. Indeed, duringpolymerization and curing of the emulsion, there is essentially a racebetween solidification of the foam structure, and coalescence of thewater droplets. An appropriate balance has to be struck such thatcoalescence of the water droplets is reduced, yet polymerization andcuring of the foam structure can be carried out within a reasonabletime. (While some coalescence can be tolerated if the remaining waterdroplets are very small in size, such nonuniform cell sizes in theresulting foam can adversely affect the fluid transport properties ofthe foam, especially its wicking rate.)

Reduction in the coalescence of water droplets in the HIPE emulsionleads to a smaller average cell size in the resulting foam structureafter polymerization and curing. It is believed that this resultingsmaller average cell size in the polymeric foam material is a keymechanism behind consistent formation of relatively thin, collapsedpolymeric foam materials according to the present invention. (Uniformlysmall cell sizes in the resulting foam are also believed to lead to goodabsorbency, and especially fluid transport (e.g., wicking)characteristics.) The number average cell size of the polymeric foammaterials is about 50 microns or less and is typically in the range fromabout 5 to about 50 microns, preferably from about 5 to about 40microns, most preferably from about 5 to about 35 microns, when preparedunder conditions that reduce coalescence of water droplets in the HIPEemulsion. Techniques for consistently reducing coalescence of waterdroplets in the HIPE emulsion will be discussed in greater detail in thefollowing description of the emulsion formation andpolymerization/curing steps for obtaining collapsed polymeric foams:

1. Formation of HIPE Emulsion

The HIPE emulsion is formed by combining the oil phase components withthe water phase components in the previously specified weight ratios.The oil phase will contain the previously specified essential componentssuch as the requisite monomers, comonomers, crosslinkers andemulsifiers, and can also contain optional components such as solventsand polymerization initiators. The water phase used will contain thepreviously specified electrolytes as an essential component and can alsocontain optional components such as water-soluble emulsifiers, and/orpolymerization initiators.

The HIPE emulsion can be formed from the combined oil and water phasesby subjecting these combined phases to shear agitation. Shear agitationis generally applied to the extent and for a time period necessary toform a stable emulsion from the combined oil and water phases. Such aprocess can be conducted in either batchwise or continuous fashion andis generally carried out under conditions suitable for forming anemulsion wherein the water phase droplets are dispersed to such anextent that the resulting polymeric foam will have the requisite porevolume and other structural characteristics. Emulsification of the oiland water phase combination will frequently involve the use of a mixingor agitation device such as a pin impeller.

One preferred method of forming HIPE emulsions which can be employedherein involves a continuous process for combining and emulsifying therequisite oil and water phases. In such a process, a liquid streamcomprising the oil phase is formed and provided at a flow rate rangingfrom about 0.08 to about 1.5 mL/sec. Concurrently, a liquid streamcomprising the water phase is also formed and provided at a flow rateranging from about 4 to about 50 mL/sec. At flow rates within theforegoing ranges, these two streams are then combined in a suitablemixing chamber or zone in a manner such that the requisite water to oilphase weight ratios as previously specified are approached, reached andmaintained.

In the mixing chamber or zone, the combined streams are generallysubjected to shear agitation as provided, for example, by a pin impellerof suitable configuration and dimensions. Shear will typically beapplied to the extent of from about 1000 to about 2500 sec.⁻¹. Residencetimes in the mixing chamber will frequently range from about 5 to about30 seconds. Once formed, the stable HIPE emulsion in liquid form can bewithdrawn from the mixing chamber or zone at a flow rate of from about 4to about 52 mL/sec. This preferred method for forming HIPE emulsions viaa continuous process is described in greater detail in U.S. Pat. No.5,149,720 (DesMarais et al), issued Sep. 22, 1992, which is incorporatedby reference.

In consistently reducing the coalescence of the water droplets presentin the HIPE emulsion, it is particularly preferred to use certain typesof emulsifier systems in the oil phase, especially if the HIPE emulsionis to be polymerized or cured at temperatures above about 50° C. Thesepreferred emulsifier systems comprise a combination of sorbitan laurate(e.g., SPAN® 20), and certain polyglycerol fatty acid esters (PGEs) asco-emulsifiers. The weight ratio of sorbitan laurate to PGE is usuallywithin the range of from about 10:1 to about 1:10. Preferably, thisweight ratio is in the range of from about 4:1 to about 1:1.

The PGEs especially useful as co-emulsifiers with sorbitan laurate areusually prepared from polyglycerols characterized by high levels oflinear (i.e., acyclic) diglycerols, reduced levels of tri- or higherpolyglycerols, and reduced levels of cyclic diglycerols. Suitablepolyglycerol reactants (weight basis) usually have a linear diglycerollevel of at least about 60% (typical range of from about 60 to about90%), a tri- or higher polyglycerol level of no more than about 40%(typical range of from about 10 to about 40%), and a cyclic diglycerollevel of no more than about 10% (typical range of from 0 to about 10%).Preferably, these polyglycerols have a linear diglycerol level of fromabout 60 to about 80%, a tri- or higher polyglycerol level of from about20 to about 40%, and a cyclic diglycerol level of no more than about10%. (A method for determining the polyglycerol distribution is setforth hereafter in the PGE ANALYTICAL METHODS section.)

PGEs especially useful as co-emulsifiers with sorbitan laurate are alsoprepared from fatty acid reactants characterized by fatty acidcompositions having high levels of combined C₁₂ and C₁₄ saturated fattyacids, and reduced levels of other fatty acids. Suitable fatty acidreactants have fatty acid compositions where the combined level of C₁₂and C₁₄ saturated fatty acids is at least about 40% (typical range offrom about 40 to about 85%), the level of C₁₆ saturated fatty acid is nomore than about 25% (typical range of from about 5 to about 25%), thecombined level of C₁₈ or higher saturated fatty acids is no more thanabout 10% (typical range of from about 2 to about 10%), the combinedlevel of C₁₀ or lower fatty acids is no more than about 10% (typicalrange of from about 0.3 to about 10%), the balance of other fatty acidsbeing primarily C₁₈ monounsaturated fatty acids. Preferably, the fattyacid composition of these fatty acid reactants is at least about 65%combined C₁₂ and C₁₄ saturated fatty acids (typical range of from about65 to about 75%), no more than about 15% C₁₆ saturated fatty acid(typical range of from about 10 to about 15%), no more than about 4%combined C₁₈ or higher saturated fatty acids (typical range of fromabout 2 to about 4%), and no more than about 3% C₁₀ or lower fatty acids(typical range of from about 0.3 to about 3%). (A method for determiningthe fatty acid composition is set forth hereafter in the PGE ANALYTICALMETHODS section.)

PGEs useful as co-emulsifiers with sorbitan laurate are also usuallycharacterized as imparting a minimum oil/water interfacial tension(IFT), where the oil phase contains monomers used in the HIPE emulsionand the water phase contains calcium chloride. Suitable PGEco-emulsifiers usually impart a minimum oil/water IFT of at least about0.06 dynes/cm, with a typical range of from about 0.06 to about 1.0dynes/cm. Especially preferred PGEs impart a minimum oil/water IFT of atleast about 0.09 dynes/cm, with a typical range of from about 0.09 toabout 0.3 dynes/cm. (A method for measuring the IFT of these PGEs is setforth hereafter in the PGE ANALYTICAL METHODS section.)

PGEs useful as coemulsifiers with sorbitan monolaurate can be preparedby methods well known in the art. See, for example, U.S. Pat. No.3,637,774 (Babayan et al), issued Jan. 25, 1972, and McIntyre,"Polyglycerol Esters," J. Am. Oil Chem. Soc., Vol. 56, No. 11 (1979),pp. 835A-840A, which are incorporated by reference and which describemethods for preparing polyglycerols and converting them to PGEs. PGEsare typically prepared by esterifying polyglycerols with fatty acids.Appropriate combinations of polyglycerols can be prepared by mixingpolyglycerols obtained from commercial sources or synthesized usingknown methods, such as those described in U.S. Pat. No. 3,637,774.Appropriate combinations of fatty acids can be prepared by mixing fattyacids and/or mixtures of fatty acids obtained from commercial sources.In making PGEs useful as co-emulsifiers, the weight ratio ofpolyglycerol to fatty acid is usually from about 50:50 to 70:30,preferably from about 60:40 to about 70:30.

Typical reaction conditions for preparing suitable PGE co-emulsifiersinvolve esterifying the polyglycerols with fatty acids in the presenceof 0.1-0.2% sodium hydroxide as the esterification catalyst. Thereaction is initiated at atmospheric pressure at about 210°-220° C.,under mechanical agitation and nitrogen sparging. As the reactionprogresses, the free fatty acids diminish and the vacuum is graduallyincreased to about 8 mm Hg. When the free fatty acid level decreases toless than about 0.5%, the catalyst is then neutralized with a phosphoricacid solution and the reaction mixture rapidly cooled to about 60° C.This crude reaction mixture can then be subjected to settling or otherconventional purification steps (e.g., to reduce the level unreactedpolyglycerol) to yield the desired PGEs.

2. Polymerization/Curing of the HIPE Emulsion

The HIPE emulsion formed will generally be collected or poured in asuitable reaction vessel, container or region to be polymerized orcured. In one embodiment herein, the reaction vessel comprises a tubconstructed of polyethylene from which the eventually polymerized/curedsolid foam material can be easily removed for further processing afterpolymerization/curing has been carried out to the extent desired. It isusually preferred that the temperature at which the HIPE emulsion ispoured into the vessel be approximately the same as thepolymerization/curing temperature.

Polymerization/curing conditions to which the HIPE emulsion will besubjected will vary depending upon the monomer and other makeup of theoil and water phases of the emulsion, especially the emulsifier systemsused, and the type and amounts of polymerization initiators utilized.Frequently, however, polymerization/curing conditions will comprisemaintenance of the HIPE emulsion at elevated temperatures above about30° C., more preferably above about 35° C., for a time period rangingfrom about 4 to about 24 hours, more preferably from about 4 to about 18hours.

In reducing coalescence of water droplets in the HIPE emulsion, it isparticularly preferred to carry out the polymerization/curing atrelatively lower temperatures, especially if the preferred combinationof sorbitan laurate and PGE co-emulsifiers is not used in preparing theHIPE emulsion. In these situations, suitable lower polymerization/curingtemperatures are in the range of from about 30° to about 50° C.,preferably from about 35° to about 45° C., and most preferably about 40°C. If polymerization/curing is carried out at temperatures much aboveabout 50° C., the thermal stress on the emulsion can cause the waterdroplets present to aggregate and coalesce, thus forming much largercells in the resulting polymeric foam, especially if the preferredcombination of sorbitan laurate and PGE co-emulsifiers is not used inpreparing the HIPE emulsion. This can lead to polymeric foams thatcannot remain in a collapsed, unexpanded state after dewatering.

A bulk solid polymeric foam is typically obtained when the HIPE emulsionis polymerized/cured in a reaction vessel, such as a tub. This bulkpolymerized HIPE foam is typically cut or sliced into a sheet-like form.Sheets of polymerized HIPE foam are easier to process during subsequenttreating/washing and dewatering steps, as well as to prepare the HIPEfoam for use in absorbent articles. The bulk polymerized HIPE foam istypically cut/sliced to provide a cut caliper in the range of from about0.08 to about 2.5 cm. During subsequent dewatering, this typically leadsto collapsed HIPE foams having a caliper in the range of from about0.008 to about 1.25 cm.

3. Treating/Washing HIPE Foam

The solid polymerized HIPE foam which is formed will generally be aflexible, open-cell porous structure having its cells filled with theresidual water phase material used to prepare the HIPE emulsion. Thisresidual water phase material, which generally comprises an aqueoussolution of electrolyte, residual emulsifier, and polymerizationinitiator, should be at least partially removed from the foam structureat this point prior to further processing and use of the foam. Removalof the original water phase material will usually be carried out bycompressing the foam structure to squeeze out residual liquid and/or bywashing the foam structure with water or other aqueous washingsolutions. Frequently several compressing and washing steps, e.g., from2 to 4 cycles, will be utilized.

After the original water phase material has been removed from the foamstructure to the extent required, the HIPE foam, if needed, can betreated, e.g., by continued washing, with an aqueous solution of asuitable hydrophilizing agent and/or hydratable salt. Hydrophilizingagents and hydratable salts which can be employed have been previouslydescribed and include sorbitan laurate (e.g., SPAN 20) and calciumchloride. As noted, treatment of the HIPE foam structure with thehydrophilizing agent/hydratable salt solution continues, if necessary,until the desired amount of hydrophilizing agent/hydratable salt hasbeen incorporated and until the foam exhibits a desired adhesion tensionvalue for any test liquid of choice.

4. Foam Dewatering

After the HIPE foam has been treated/washed to the extent necessary torender the eventually dried foam suitably hydrophilic, and optionally toincorporate a sufficient amount of a hydratable salt, preferably calciumchloride, the foam will generally be dewatered. Dewatering can bebrought about by compressing the foam (preferably in the z-direction) tosqueeze out residual water, by subjecting the foam, or the watertherein, to elevated temperatures, e.g., thermal drying at temperaturesfrom about 60° C. to about 200° C., or to microwave treatment, by vacuumdewatering or by a combination of compression and thermaldrying/microwave/vacuum dewatering techniques. The dewatering step ofHIPE foam processing will generally be carried out until the HIPE foamis ready for use and is as dry as practical. Frequently such compressiondewatered foams will have a water (moisture) content of from about 50 toabout 500%, more preferably from about 50 to about 200%, by weight on adry weight basis. Subsequently, the compressed foams can be thermallydried (e.g., by heating) to a moisture content of from about 5 to about40%, more preferably from about 5 to about 15%, on a dry weight basis.The resulting compressed/dried foam will be in a collapsed, unexpandedstate.

VI. Absorbent Articles

The collapsed polymeric foam materials of the present invention can beused as at least a portion of the absorbent structures (e.g., absorbentcores) for various absorbent articles. By "absorbent article" herein ismeant a consumer product which is capable of absorbing significantquantities of urine or other fluids (i.e., liquids), like aqueous fecalmatter (runny bowel movements), discharged by an incontinent wearer oruser of the article. Examples of such absorbent articles includedisposable diapers, incontinence garments, catamenials such as tamponsand sanitary napkins, disposable training pants, bed pads, and the like.The absorbent foam structures herein are particularly suitable for usein articles such as diapers, incontinence pads or garments, clothingshields, and the like.

In its simplest form, an absorbent article of the present invention needonly include a backing sheet, typically relatively liquid-impervious,and one or more absorbent foam structures associated with this backingsheet. The absorbent foam structure and the backing sheet will beassociated in such a manner that the absorbent foam structure issituated between the backing sheet and the fluid discharge region of thewearer of the absorbent article. Liquid impervious backing sheets cancomprise any material, for example polyethylene or polypropylene, havinga caliper of about 1.5 mils (0.038 mm), which will help retain fluidwithin the absorbent article.

More conventionally, the absorbent articles herein will also include aliquid-pervious topsheet element which covers the side of the absorbentarticle that touches the skin of the wearer. In this configuration, thearticle includes an absorbent core comprising one or more absorbent foamstructures of the present invention positioned between the backing sheetand the topsheet. Liquid-pervious topsheets can comprise any materialsuch as polyester, polyolefin, rayon and the like which is substantiallyporous and permits body fluid to readily pass therethrough and into theunderlying absorbent core. The topsheet material will preferably have noaffinity for holding aqueous body fluids in the area of contact betweenthe topsheet and the wearer's skin.

The absorbent core of the absorbent article embodiments of thisinvention can consist solely of one or more of the foam structuresherein. For example, the absorbent core can comprise a single unitarypiece of foam shaped as desired or needed to best fit the type ofabsorbent article in which it is to be used. Alternatively, theabsorbent core can comprise a plurality of foam pieces or particleswhich can be adhesively bonded together or which can simply beconstrained into an unbonded aggregate held together by an overwrappingof envelope tissue or by means of the topsheet and backing sheet of theabsorbent article.

The absorbent core of the absorbent articles herein can also compriseother, e.g., conventional, elements or materials in addition to one ormore absorbent foam structures of the present invention. For example,absorbent articles herein can utilize an absorbent core which comprisesa combination, e.g., an air-laid mixture, of particles or pieces of theabsorbent foam structures herein and conventional absorbent materialssuch as a) wood pulp or other cellulosic fibers, and/or, b) particles orfibers of polymeric gelling agents.

In one embodiment involving a combination of the absorbent foam hereinand other absorbent materials, the absorbent articles herein can employa multi-layer absorbent core configuration wherein a core layercontaining one or more foam structures of this invention can be used incombination with one or more additional separate core layers comprisingconventional absorbent structures or materials. Such conventionalabsorbent structures or materials, for example, can include air-laid orwet-laid webs of wood pulp or other cellulosic fibers. Such conventionalstructures can also comprise conventional, e.g., large cell, absorbentfoams or even sponges. The conventional absorbent structures used withthe absorbent foam herein can also contain, for example up to 80% byweight, of particles or fibers of polymeric gelling agent of the typecommonly used in absorbent articles that are to acquire and retainaqueous body fluids. Polymeric gelling agents of this type and their usein absorbent articles are more fully described in Reissue U.S. Pat. No.Re. 32,649 (Brandt et al), reissued Apr. 19, 1988, which is incorporatedby reference.

One preferred type of absorbent article herein is one which utilizes amulti-layer absorbent core having fluid handling layer positioned in thefluid discharge region of the wearer of the article. This fluid-handlinglayer can be in the form of a high loft nonwoven, but is preferably inthe form of a fluid acquisition/distribution layer comprising a layer ofmodified cellulosic fibers, e.g., stiffened curled cellulosic fibers,and optionally up to about 10% by weight of this fluidacquisition/distribution layer of polymeric gelling agent. The modifiedcellulosic fibers used in the fluid acquisition/distribution layer ofsuch a preferred absorbent article are preferably wood pulp fibers whichhave been stiffened and curled by means of chemical and/or thermaltreatment. Such modified cellulosic fibers are of the same type as areemployed in the absorbent articles described in U.S. Pat. No. 4,935,622(Lash et al), issued Jun. 19, 1990, which is incorporated by reference.

These multi-layer absorbent cores also comprise a second, i.e., lower,fluid storage/redistribution layer comprising a foam structure of thepresent invention. For purposes of this invention, an "upper" layer of amulti-layer absorbent core is a layer which is relatively closer to thebody of the wearer, e.g., the layer closest to the article topsheet. Theterm "lower" layer conversely means a layer of a multi-layer absorbentcore which is relatively further away from the body of the wearer, e.g.,the layer closest to the article backsheet. This lower fluidstorage/redistribution layer is typically positioned within theabsorbent core so as to underlie the (upper) fluid-handling layer and bein fluid communication therewith. Absorbent articles which can utilizethe absorbent foam structures of this invention in a lower fluidstorage/redistribution layer underlying an upper fluidacquisition/distribution layer containing stiffened curled cellulosicfibers are described in greater detail in the U.S. Pat. No. 5,147,345(Young et al), issued Sep. 15, 1992 which is incorporated by reference.

As indicated hereinbefore, the fluid handling and mechanicalcharacteristics of the specific absorbent foam structures herein rendersuch structures especially suitable for use in absorbent articles in theform of disposable diapers. Disposable diapers comprising the absorbentfoam structures of the present invention can be made by usingconventional diaper making techniques, but by replacing or supplementingthe wood pulp fiber web ("airfelt") or modified cellulosic coreabsorbents typically used in conventional diapers with one or more foamstructures of the present invention. Foam structures of this inventioncan thus be used in diapers in single layer or, as noted hereinbefore,in various multiple layer core configurations. Articles in the form ofdisposable diapers are more fully described in U.S. Patent Re 26,151(Duncan et al), issued Jan. 31, 1967; U.S. Pat. No. 3,592,194 (Duncan),issued Jul. 13, 1971; U.S. Pat. No. 3,489,148 (Duncan et al), issuedJan. 13, 1970; U.S. Pat. No. 3,860,003, issued Jan. 14, 1975; and U.S.Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989; all of whichare incorporated by reference.

A preferred disposable diaper embodiment of this invention isillustrated by FIG. 5 of the drawings. Such a diaper includes anabsorbent core 50, comprising an upper fluid acquisition layer 51, andan underlying fluid storage/distribution layer 52 comprising anabsorbent foam structure of this invention. A topsheet 53 is superposedand co-extensive with one face of the core, and a liquid imperviousbacksheet 54 is superposed and coextensive with the face of the coreopposite the face covered by the topsheet. The backsheet most preferablyhas a width greater than that of the core thereby providing sidemarginal portions of the backsheet which extend beyond the core. Thediaper is preferably constructed in an hourglass configuration.

Another preferred type of absorbent article which can utilize theabsorbent foam structures of the present invention comprisesform-fitting .products such as training pants. Such form-fittingarticles will generally include a nonwoven, flexible substrate fashionedinto a chassis in the form of briefs or shorts. An absorbent foamstructure according to the present invention can then be affixed in thecrotch area of such a chassis in order to serve as an absorbent "core".This absorbent core will frequently be over-wrapped with envelope tissueor other liquid pervious, nonwoven material. Such core overwrapping thusserves as the "topsheet" for the form-fitting absorbent article.

The flexible substrate which forms the chassis of the form-fittingarticle can comprise cloth or paper or other kinds of nonwoven substrateor formed films and can be elasticized or otherwise stretchable. Legbands or waist bands of such training pants articles can be elasticizedin conventional fashion to improve fit of the article. Such a substratewill generally be rendered relatively liquid-impervious, or at least notreadily liquid-pervious, by treating or coating one surface thereof orby laminating this flexible substrate with another relativelyliquid-impervious substrate to thereby render the total chassisrelatively liquid-impervious. In this instance, the chassis itselfserves as the "backsheet" for the form-fitting article. Typical trainingpants products of this kind are described in U.S. Pat. No. 4,619,649(Roberts), issued Oct. 28, 1986, which is incorporated by reference.

A typical form-fitting article in the form of a disposable trainingpants product is shown in FIG. 6 of the drawings. Such a productcomprises an outer layer 60 affixed to a lining layer 61 by adhesionalong the peripheral zones thereof. For example, the inner lining 61 canbe affixed to the outer layer 60, along the periphery of one leg bandarea 62, along the periphery of the other leg band area 63, and alongthe periphery of waistband area 64. Affixed to the crotch area of thearticle is a generally rectangular absorbent core 65 comprising anabsorbent foam structure of the present invention.

TEST METHODS

In describing the present invention, a number of characteristics of theHIPE foam absorbent structures are set forth. Where reported, thesecharacteristics can be determined using the following test fluids andtest methods.

I) Test Fluids and Foam Sample Preparation

A) Test Fluid--Synthetic Urine

Several of the measurements described in the tests herein involve theuse of a test fluid such as synthetic urine, ethanol, or 2-propanol(isopropyl alcohol). The synthetic urine utilized in a number of thetests described hereafter is made from a commercially availablesynthetic urine preparation manufactured by Jayco Pharmaceuticals(Mechanicsburg, Pa., 17055). This Jayco synthetic urine made from thepreparation comprises KCl, 0.2%; Na₂ SO₄, 0.2%; NH₄ H₂ PO₄, 0.085%;(NH₄)₂ HPO₄, 0.015%; CaCl₂ *2H₂ O, 0.025%; and MgCl₂ *6H₂ O, 0.05%.(weight %'s) The synthetic urine samples are prepared according to thelabel instructions using distilled water. To aid dissolution, the Jaycosalt mixture is slowly added to the water. The sample is filtered ifnecessary to remove any particulates. Any unused synthetic urine isdiscarded after one week. To improve visibility of the fluid, 5 drops ofblue food color can be added per liter of synthetic urine solution. TheJayco synthetic urine utilized has a surface tension of 65±5 dynes/cm.

B) Foam Sample Preparation

A number of the following tests involve the preparation and testing offoam samples of a particular specified size. Unless otherwise specified,foam samples of the requisite size should be cut from larger blocks offoam using a sharp reciprocating knife saw. Use of this or equivalenttype of foam cutting device serves to substantially eliminate sampleedge flaws which could have an adverse impact on certain of themeasurements made in carrying out the several test procedures hereafterset forth.

Sample size specification will also generally include a dimension forsample caliper or thickness. Caliper or thickness measurements forpurposes of the present invention should be made when the foam sample isunder a confining pressure of 0.05 psi (350 Pa). All measurements offoam density and dry weight are usually carried out after the foamsample has been water washed and dried, as described hereafter.

II) Determination of Properties, Features or Characteristics of Foam

A. Collapsed State

1) Expansion Pressure

This test directly measures the stored energy in the collapsed foam. Thestored energy of the collapsed foam is released when it is flooded withan amount of water greater than its free absorbent capacity. Expansionpressure is measured while the fully wetted foam is being held bycompressive forces at its collapsed caliper (thickness).

To conduct this test, a 23.8 mm. diameter cylinder of the collapsed foamis carefully cut out using a punch. The caliper of this cut sample ismeasured with a strain gauge (e.g., Ono-Sokki Model EG-225) to thenearest 0.005 mm. In performing the stress relaxation test, aRheometrics Model RSA II is used having a parallel plate assemblycapable of retaining liquid. This parallel plate assembly comprises abottom cup plate having a cylindrical chamber with an inside diameter of29 mm., and a top plate having a circular member with a diameter of 25mm.

The cut, dry sample is placed within the chamber of the bottom cup plateand centered under the circular member of the top plate. The entireassembly/sample is then equilibrated at 88° F. (31.1° C.) for at least10 minutes, with the top plate being adjusted to rest on the cut samplewith a force of about 10 g. The Rheometrics Model RSA II is programmedto run a stress-relaxation test at 0.5% strain (in compression) at 88°F. (31.1° C.). While monitoring stress as a function of time, enoughwater having a temperature of 88° F. (31.1° C.) is quickly added to thebottom cup plate using a syringe to insure complete saturation of thecut sample (e.g., 3 mL in 1 second). The pressure the cut sample exertson the plates as it tries to expand (i.e. its expansion pressure) isrecorded for at least 15 minutes after the point at which the water isadded; the value at 5 minutes is recorded as the expansion pressure ofthe cut sample.

2) Capillary Suction Specific Surface Area

Capillary suction specific surface area of the foam can be determinedfrom the equilibrium weight uptake of a test liquid of known low surfacetension. In this instance, absolute ethanol (flash point is 10° C.) isused.

To conduct the test, a tared foam sample strip of suitable dimensions(e.g., ≧35 cm long×2 cm wide×0.25 cm thick) is equilibrated at 22±2° C.,is positioned vertically and at one end is immersed 1-2 mm into areservoir of the ethanol using a lab jack. The ethanol is allowed towick up the foam strip to its equilibrium height which should be lessthan the sample length. The ethanol-containing strip is then weighed todetermine the weight of total ethanol uptake. During this procedure thesample should be shielded, for example with a capped glass cylinder, toprevent ethanol evaporation. The ethanol is then allowed to evaporatefrom the foam sample which is then water washed, dried and weighed.

Specific surface area of the foam sample can be calculated from thefollowing formula: ##EQU3## where S_(c) =capillary suction specificsurface area in cm² /gm; M_(e) =mass of liquid uptake of ethanol in gms;G=the gravitational constant which is 980 cm/sec² ; L_(n) =total lengthof wet sample in cm; M_(n) =mass of dry sample in gm; and γ_(e) =surfacetension of ethanol which is 22.3 dynes/cm. Values obtained can then bedivided by 10000 cm² /m² to provide capillary suction specific surfacearea in m² /g.

3) Foam Density

One procedure which can be used to determine foam density is thatdescribed in ASTM Method No. D3574-86, Test A, which is designedprimarily for the testing of urethane foams but which can also beutilized for measuring the density of the preferred HIPE-type foams ofthe present invention. In particular, density measurements madeaccording to this ASTM procedure are carried out on foam samples whichhave been preconditioned in a certain manner as specified in that test.

Density is determined by measuring both the dry mass of a given foamsample (after water washing and drying) and its volume at 22±2° C.Volume determinations on larger foam samples are calculated frommeasurements of the sample dimensions made under no confining pressure.Dimensions of smaller foam samples can be measured using a dial-typegauge using a pressure on the dial foot of 350 Pa (0.05 psi).

Density is calculated as mass per unit volume. For purposes of thisinvention, density is generally expressed in terms of g/cm³.

B. Expanded State

1) Density Upon Saturation with Synthetic Urine

In this measurement, the foam sample is saturated at 88° F. (31.1° C.)to its free absorbent capacity with Jayco synthetic urine. The volume ismeasured in this fully expanded state and the dry mass of the sample ismeasured after water washing and drying. The density upon saturationwith synthetic urine is thus calculated as dry mass per wet volume,expressed in terms of g/cm³.

2) Available Pore Volume

A procedure for determining available pore volume involves themeasurement of the amount of 2-propanol (flash point 12° C.) which canbe introduced into the structure of an absorbent foam sample. Equipmentand materials used in making such a measurement are equilibrated at22±2° C. Measurements are also performed at this temperature.

Dry foam samples are cut into 1 in² (6.5 cm²) circular surface area×0.1inch (0.25 cm) thick cylinders or the equivalent. Such cylindricalsamples can be prepared by using a sharp punch 1.13 inches (2.87 cm) indiameter on a 0.1 inch (0.25 cm) sheet of foam. The dry foam samples(after water washing and drying) are each weighed to determine a dryweight (DW). Three of such samples are weighed to determine an averagedry weight (DW).

The Measured Free Capacity (MFC) of these samples is then determined bythe following steps:

a) The foam samples are immersed in the 2-propanol in a crystallizingdish and allowed to saturate. At this point the sample may be squeezed afew times to expel air.

b) Each sample is removed without squeezing 2-propanol out of it. Excessfluid is allowed to drip off of the sample in the flat position forabout 30 seconds. Each sample is then weighed wet to determine a wetweight (WW).

c) Steps a) and b) are repeated two more times and an average wet weight(WW) is calculated.

Measured Free Capacity (MFC, g/g) is the weight of 2-propanol in thesaturated foam per unit mass of dry foam. MFC is calculated according tothe formula ##EQU4##

Available pore volume is then calculated by dividing the MFC of the foamfor 2-propanol by the density of 2-propanol which is 0.785 g/mL. Thisgives an available pore volume for the foam in mL/g.

3) Resistance to Compression Deflection

Resistance to compression deflection can be quantified for purposes ofthis invention by measuring the amount of strain (% caliper reduction)produced in a foam sample, which has been saturated and fully expandedwith synthetic urine, after stress in the form of a 0.74 psi (5.1 kPa)confining pressure has been applied to the sample.

The foam samples, Jayco synthetic urine and equipment used to makemeasurements are all equilibrated to a temperature of 88° F. (31.1° C.).Measurements are also performed at this temperature.

A foam sample sheet in its collapsed state is saturated to its freeabsorbent capacity with Jayco synthetic urine. After 2 minutes, acylinder having a 1 in² (6.5 cm²) circular surface area is punched outof the saturated, fully expanded sheet. A dial-type gauge suitable formaking caliper measurements is positioned on the sample. Any gaugefitted with a foot having a circular surface area of at least 1 in² (6.5cm²) and capable of measuring caliper dimensions to 0.001 in (0.025 mm)can be employed. Examples of such gauges are an Ames model 482 (AmesCo.; Waltham, Mass.) or an Ono-Sokki model EG-225 (Ono-Sokki Co., Ltd.;Japan).

A force is then applied to the foot so that the saturated foam sample onthe screen is subjected to a confining pressure of 0.74 psi (5.1 kPa)for 15 minutes. At the end of this time, the dial gauge is used tomeasure the change in sample caliper which occurs as a consequence ofthe application of the confining pressure. From the initial and finalcaliper measurements, a percent strain induced can be calculated for thesample.

4) Recovery From Compression Deflection

To test recovery from compression deflection, foam samples similar tothose prepared for the Resistance to Compression Deflection test (seeII(B)(3) above) are used.

Using a dial-type gauge, a test sample saturated to its free absorbentcapacity in Jayco synthetic urine at 88° F. (31.1° C.) is compressedwithin 10 seconds to 50% of its original thickness and maintained in thecompressed state for 1 minute. The pressure is then released, and thefoam is allowed to recover thickness for 1 minute in the presence of theexpelled fluid. The percent recovery is based on the original height ofthe uncompressed foam.

C. Collapsed or Expanded States

1) Flexibility

Foam flexibility can be quantified by referencing a test procedure whichis a modification of the ASTM D 3574-86, 3.3 test used to determineflexibility of cellular organic polymeric foam products. Such a modifiedtest utilizes a foam sample which is 7×0.8×0.8 cm when saturated to itsfree absorbent capacity with Jayco synthetic urine at 88° F. (31.1° C.).It is important that the cutting process used to make these samples doesnot introduce edge defects in the foam strip. The syntheticurine-saturated foam strip is bent around a 0.8 cm diameter cylindricalmandrel at a uniform rate of 1 lap in 5 seconds until the ends of thestrip meet. The foam is considered flexible if it does not tear or breakduring this test, i.e., if it passes one bending cycle.

D. Determination of Fluid Handling Characteristics

1) Free Absorbent Capacity

In this test, a foam sample is saturated at 88° F. (31.1° C.) with Jaycosynthetic urine. The same procedure described in II(B)(2) above forAvailable Pore Volume is then used to measure the no-load (free)absorbent capacity.

2) Vertical Wicking Rate and Vertical Wicking Absorbent Capacity

Vertical wicking rate and vertical wicking absorbent capacity aremeasures of the ability of a dry foam to wick fluid vertically from areservoir. The time required for the fluid front to wick through a 5 cmvertical length of a strip of foam is measured to give a verticalwicking rate. After fluid wicks to its equilibrium height, the amount offluid held by the foam strip at a particular vertical wicking height(e.g., 4.5 inches or 11.4 cm) is determined to give a vertical wickingabsorbent capacity.

Jayco synthetic urine colored with blue food coloring is used in thefollowing methods to determine vertical wicking rate and verticalwicking absorbent capacity. In this test procedure, the materials areequilibrated at 37° C. and the test is performed at the sametemperature.

A strip of foam approximately 70 cm long×2 cm wide×0.25 cm thick issupported vertically with one end immersed 1 to 2 mm into a reservoir ofsynthetic urine. The liquid is allowed to wick up the foam strip to itsequilibrium height (e.g., about 18 hours), which should be less than thesample length. During this procedure, the sample should be shielded, forexample with a capped glass cylinder, to prevent evaporation.

The time needed to wick 5 cm is used as a measure of vertical wickingrate. The equilibrium wet weight can be recorded and used to calculateAdhesion Tension, as described below.

The sample is quickly removed and placed on a nonabsorbent surface whereit is cut into 1 inch (2.54 cm) pieces using a tool sharp enough not tocompress the foam sample. Each piece is weighed, washed with water,dried and then reweighed. The absorbent capacity is calculated for eachpiece. The absorbent capacity of the 1 inch segment centered at 4.5inches (11.4 cm) wicking height is the parameter most desirablydetermined.

3) Adhesion Tension

The adhesion tension exhibited by hydrophilized foam samples whichimbibe test fluids via capillary suction is the product of the surfacetension, γ, of the test fluid times the cosine of the contact angle, θ,exhibited by the test fluid in contact with the interior surfaces of thefoam sample. Adhesion tension can be determined experimentally bymeasuring the equilibrium weight uptake by capillary suction exhibitedby two test samples of the same foam using two different test liquids.In the first step of such a procedure, specific surface area of the foamsample is determined using ethanol as the test fluid as described inII(A)(2) above for Capillary Suction Specific Surface Area.

The capillary suction uptake procedure is then repeated in identicalmanner to the ethanol procedure except that: (a) Jayco synthetic urineis used as the test fluid; (b) the test is carried out at 37° C.; and(c) a foam sample strip≧70 cm long×2 cm wide×0.25 cm thick is used. Thecontact angle of the synthetic urine can then be calculated as followsfrom the known specific surface area and the synthetic urine uptakedata: ##EQU5## where θ_(U) =contact angle of Jayco synthetic urine indegrees; M_(U) =mass of liquid uptake of Jayco synthetic urine in gms;G=gravitational constant which is 980 cm/sec² ; M_(N) =mass of dry foamsample in gm; γ_(U) =surface tension of Jayco urine which is ˜65dynes/cm; S_(c) =specific surface area of the foam sample in cm² /gm asdetermined by the ethanol uptake procedure; and L_(n) =length of the wetfoam sample in cm.

When a surfactant is present (on the foam sample surfaces and/or in theadvancing test liquid), characterization of the advancing liquid frontis defined by applying the adhesion tension (AT) equation: ##EQU6##wherein M_(T) is the mass of the test liquid taken up by the foamsample, and G, L_(N), M_(N), and S_(c) are as defined before. [SeeHodgson and Berg, J. Coll. Int. Sci., 121(1), 1988, pp. 22-31]

In determining adhesion tension for any given test liquid, no assumptionis made of the numerical value of the surface tension at any point intime so that possible changes in surfactant concentration on the samplesurfaces and/or in the advancing liquid during wicking are immaterial.The experimental value of adhesion tension (γ cosθ) is especially usefulwhen viewed as a percentage of the maximum adhesion tension which is thesurface tension of the test liquid (e.g., the maximum adhesion tensionusing Jayco synthetic urine would be [65±5] [cos 0°]=65±5 dynes/cm).

PGE ANALYTICAL METHODS

A. Interfacial tension (IFT) method (Spinning Drop)

IFT can be measured using a Kruss SITE 04 Spinning Drop Tensiometer orany equivalent spinning drop tensiometer operating under the sameprinciples. See Cayias et al, Absorption at Interfaces, edited byMittal, ACS Symposium Series 8 (1975), pp. 234-47, and Aveyard et al, J.Chem. Soc. Faraday Trans., Vol. 1 (1981), pp. 2155-68, for generaldescriptions of spinning drop IFT methods. For each IFT measurement, adrop of the oil phase containing the PGE (typically 5 microliters) isinjected into the spinning capillary tube of the tensiometer whichcontains the aqueous phase pre-equilibrated to 50° C. The spinning rateof the tube is increased sufficiently to elongate the drop into acylindrical shape such that its ratio of length:diameter is greater than4:1. The elongated drop is allowed to equilibrate at 50° C. untilequilibrium is reached (i.e. no further change in drop diameter) oruntil approximately 30 minutes have elapsed. Preferably, the elongateddrop is of uniform diameter (except at the hemispherical ends) and itsinterface with the aqueous solution is essentially free of depositedmaterial and/or smaller droplets. If not, then a region of the elongateddrop that is essentially free of deposits, has a length:diameter ratioof greater than 4:1 and a minimum diameter (if regions of differentdiameter are present) is chosen for the measurement. The interfacialtension (γ) is calculated from the measured radius (r, 1/2 thecylindrical diameter), the angular velocity (ω; 2πn, where n is thefrequency of rotation of the tube), and the density difference betweenthe oil and aqueous phases (Δ ρ), using the following equation:

    γ=0.25 r.sup.3 Δρω.sup.2

Calibration of this system is periodically checked by measuring the IFTbetween n-octanol and water (8.5 dynes/cm at 20° C.).

The oil phase used for these measurements is typically prepared byadding 10 parts of the PGE to 100 parts of a monomer mixture containingstyrene, divinylbenzene (55% technical grade) and 2-ethylhexylacrylatein a weight ratio of 2:2:6. This mixture is mechanically agitated toeffect dissolution of the PGE. This oil phase mixture is allowed tostand overnight at ambient temperature so that materials (typically freepolyglycerols) which are insoluble or precipitate out of solution cansettle. After centrifugation, the supernatant is separated from anysediment. The supernatant is used as is and diluted (sequentially ifnecessary) with additional monomer mixture to yield emulsifier solutionshaving lower concentrations. In this fashion, solutions having nominalPGE concentrations ranging from approximately 9% to 0.01% or lower canbe prepared. The water phase used for this measurement is an 0.90Maqueous solution of calcium chloride (approximate pH 6) prepared bydissolving CaCl₂.2H₂ O in distilled water.

A series of IFT measurements are made by varying the concentration ofPGE in the oil phase up to at least 3%. A smooth curve is drawn througha log-log plot of IFT values as a function of PGE concentration in theoil phase. A minimum IFT value is then estimated from the curve.

B. Polyglycerol Distribution

The distribution of polyglycerols in a sample can be determined bycapillary supercritical fluid chromatography. See Chester et al, J. HighRes. Chrom. & Chrom. Commun., Vol. 9 (1986), p. 178 et seq. In thismethod, the polyglycerols in the sample are converted to the respectivetrimethylsilyl ethers by reaction withbis(trimethylsilyl)-trifluoroacetamide. The trimethylsilyl ethers areseparated and then quantified by capillary supercritical fluidchromatography using flame ionization detection. The trimethylsilylethers elute from the polydimethylsiloxane stationary phase in order ofincreasing molecular weight. Peak identities are established by couplingthe supercritical fluid chromatograph to a mass spectrometer. Therelative distribution of polyglycerol species is calculated from peakareas in the chromatogram. Weight percentages are calculated by assumingthat the flame ionization detection responds equally to all polyglycerolspecies in the sample.

C. Fatty Acid Composition (FAC)

The fatty acid composition of a sample of free fatty acids or fatty acidesters can be determined by high resolution capillary gaschromatography. See D'Alonzo et al, J. Am. Oil Chem. Soc., Vol. 58(1981), p. 215 et seq. In this method, the fatty acids in the sample areconverted to fatty acid methyl esters that are then separated andquantified by high resolution capillary gas chromatography with flameionization detection. The capillary column stationary phase (astabilized polyethylene glycol) separates the methyl esters according tochain length and degree of unsaturation. Peak identities are establishedby comparison with known fatty acid standards. The relative distributionof fatty acid species is calculated from the peak areas in thechromatogram. Weight percentages are calculated by assuming the flameionization detector responds equally to all fatty acid species in thesample.

EXAMPLES

Preparation of collapsed HIPE absorbent foams, the characteristics ofsuch collapsed foams and utilization of these collapsed absorbent foamsin disposable diapers are all illustrated by the following examples.

EXAMPLE I

This example illustrates the preparation of a collapsed HIPE foamfalling within the scope of the present invention.

Emulsion Preparation

Anhydrous calcium chloride (36.32 kg) and potassium persulfate (568 g)are dissolved in 378 liters of water. This provides the water phasestream to be used in a continuous process for forming a HIPE emulsion.

To a monomer combination comprising styrene (1600 g), divinylbenzene 55%technical grade (1600 g), and 2-ethylhexylacrylate (4800 g) is addedsorbitan laurate (960 g as SPAN® 20). After mixing, this combination ofmaterials is allowed to settle overnight. The supernatant is withdrawnand used as the oil phase in a continuous process for forming a HIPEemulsion. (About 75 g of a sticky residue is discarded.)

At an aqueous phase temperature of 48°-50° C. and an oil phasetemperature of 22° C., separate streams of the oil phase and water phaseare fed to a dynamic mixing apparatus. Thorough mixing of the combinedstreams in the dynamic mixing apparatus is achieved by means of a pinimpeller. At this scale of operation, an appropriate pin impellercomprises a cylindrical shaft of about 21.6 cm in length with a diameterof about 1.9 cm. The shaft holds 4 rows of pins, 2 rows having 17 pinsand 2 rows having 16 pins, each having a diameter of 0.5 cm extendingoutwardly from the central axis of the shaft to a length of 1.6 cm. Thepin impeller is mounted in a cylindrical sleeve which forms the dynamicmixing apparatus, and the pins have a clearance of 0.8 mm from the wallsof the cylindrical sleeve.

A spiral static mixer is mounted downstream from the dynamic mixingapparatus to provide back pressure in the dynamic mixer and to provideimproved incorporation of components into the emulsion that iseventually formed. Such a static mixer is 14 inches (35.6 cm) long witha 0.5 inch (1.3 cm) outside diameter. The static mixer is a TAHIndustries Model 070-821, modified by cutting off 2.4 inches (6.1 cm).

The combined mixing apparatus set-up is filled with oil phase and waterphase at a ratio of 2 parts water to 1 part oil. The dynamic mixingapparatus is vented to allow air to escape while filling the apparatuscompletely. The flow rates during filling are 1.127 g/sec oil phase and2.19 cm³ /sec water phase.

Once the apparatus set-up is filled, agitation is begun in the dynamicmixer, with the impeller turning at 1800 RPM. The flow rate of the waterphase is then steadily increased to a rate of 35.56 cm³ /sec over a timeperiod of 130 sec. The back pressure created by the dynamic and staticmixers at this point is 7.5 PSI (51.75 kPa). The impeller speed is thensteadily decreased to a speed of 1200 RPM over a period of 60 sec. Theback pressure drops to 4.5 PSI (31.05 kPa). At this point, the impellerspeed is instantly increased to 1800 RPM. The system back pressureremains constant thereafter at 4.5 PSI (31.05 kPa).

Polymerization of the Emulsion

The formed emulsion flowing from the static mixer at this point iscollected in Rubbermaid Economy Cold Food Storage Boxes, Model 3500.These boxes are constructed of food grade polyethylene and have nominaldimensions of 18"×26"×9" (45.7 cm×66 cm 22.9 cm). The true insidedimensions of these boxes are 15"×23"×9" (38.1 cm×58.4 cm×22.9 cm).These boxes are pretreated with a film of a solution comprising a 20%solution of SPAN® 20 in an equal weight solvent mixture of xylene and2-propanol. The solvent mixture is allowed to evaporate to leave onlythe SPAN® 20. Forty-seven liters of emulsion are collected in each box.

The emulsion-containing boxes are kept in a room maintained at 65° C.for 18 hours to bring about polymerization of the emulsion in the boxesto thereby form polymeric foam.

Foam Washing and Dewatering

After curing is complete, the wet cured foam is removed from the curingboxes. The foam at this point contains about 30-40 times the weight ofpolymerized material (30-40×) of the residual water phase containingdissolved emulsifiers, electrolyte and initiator. The foam is slicedwith a sharp reciprocating saw blade into sheets which are 0.350 inches(0.89 cm) in caliper. These sheets are then subjected to compression ina series of 3 nip rolls which gradually reduce the residual water phasecontent of the foam to about 6 times (6×) the weight of the polymerizedmaterial. At this point, the sheets are then resaturated with a 1% CaCl₂solution at 60° C., are squeezed in a nip to a water phase content ofabout 10×, resaturated with the 1% CaCl₂ solution at 60° C., and thensqueezed again in a nip to a water phase content of about 10×.

The foam sheets, which now contain about 10× of what is essentially a 1%CaCl₂ solution are passed through a final nip equipped with a vacuumslot. The last nip reduces the CaCl₂ solution content to about 5 times(5×) the weight of polymer. The foam remains compressed after the finalnip at a caliper of about 0.080 in. (0.2 cm). The foam is then dried inan air circulating oven set at about 60° C. for about three hours. Suchdrying reduces the moisture content to about 5-7% by weight ofpolymerized material. At this point, the foam sheets have a caliper ofabout 0.075 in. (0.19 cm) and are very drapeable. The foam also containsabout 11% by weight of residual sorbitan laurate emulsifier and about 5%by weight (anhydrous basis) of residual hydrated calcium chloride. Inthe collapsed state, the density of the foam is about 0.17 g/cm³. Whenexpanded in Jayco synthetic urine, its free absorbent capacity is about30.2 mL/g. The expanded foam has a capillary suction specific surfacearea of about 2.24 m² /g, a pore volume of about 31 mL/g, a numberaverage cell size of about 15 microns, an adhesion tension of about 35dynes/cm, and a vertical wicking absorbent capacity of about 26.7 mL/gor about 88% of its free absorbent capacity.

EXAMPLE II

A disposable diaper is prepared using the configuration and componentsshown in expanded and blown-apart depiction in FIG. 7. Such a diapercomprises a thermally bonded polypropylene topsheet 70, afluid-impervious polyethylene backing sheet 71, and a dual layerabsorbent core positioned between the topsheet and the backing sheet.The dual layer absorbent core comprises a modified hourglass-shaped,fluid storage/redistribution layer 72 comprising the collapsed HIPE foamof the Example I type positioned below a modified-hourglass shaped fluidacquisition layer 73. The topsheet contains two substantially parallelbarrier leg cuff strips 74 with elastic. Affixed to the diaper backsheetare two rectangular elasticized waistband members 75. Also affixed toeach end of the polyethylene backsheet are two waistshield elements 76constructed of polyethylene. Also affixed to the backsheet are twoparallel leg elastic strips 77. A sheet of polyethylene 78 is affixed tothe outside of the backsheet as a dedicated fastening surface for twopieces 79 of Y-tape which can be used to fasten the diaper around thewearer.

The acquisition layer of the diaper core comprises a 92%/8% wet-laidmixture of stiffened, twisted, curled cellulosic fibers and conventionalnon-stiffened cellulosic fibers. The stiffened, twisted, curledcellulosic fibers are made from southern softwood kraft pulp (Foleyfluff) which has been crosslinked with glutaraldehyde to the extent ofabout 2.5 mole percent on a dry fiber cellulose anhydroglucose basis.The fibers are crosslinked according to the "dry crosslinking process"as described in U.S. Pat. No. 4,822,453 (Dean et al), issued Apr. 18,1989.

These stiffened fibers are similar to the fibers having thecharacteristics described as follows in Table I.

Table I

Stiffened, Twisted, Curled Cellulose (STCC) Fibers

Type=Southern softwood kraft pulp crosslinked with glutaraldehyde to theextent of 1.41 mole percent on a dry fiber cellulose anhydroglucosebasis

Twist Count Dry=6.8 nodes/mm

Twist Count Wet: 5.1 nodes/mm

2-Propanol Retention Value=24%

Water Retention Value=37%

Curl Factor=0.63

The conventional non-stiffened cellulose fibers used in combination withthe STCC fibers are also made from Foley fluff. These non-stiffenedcellulose fibers are refined to about 200 CSF (Canadian StandardFreeness).

The acquisition layer has an average dry density of about 0.07 g/cm³, anaverage density upon saturation with synthetic urine, dry weight basis,of about 0.08 g/cm³, and an average basis weight of about 0.03 g/cm².About 13 grams of the fluid acquisition layer are used in the diapercore. The surface area of the acquisition layer is about 46.8 in² (302cm²). It has a caliper of about 0.44 cm.

The fluid storage/redistribution layer of the diaper core comprises amodified hourglass-shaped piece of collapsed HIPE foam of the typedescribed in Example I. About 13 grams of HIPE foam are used to formthis storage/distribution layer which has a surface area of about 52.5in² (339 cm²) and a caliper of about 0.1 in (0.25 cm).

A diaper having this particular core configuration exhibits especiallydesirable and efficient utilization of the core for holding dischargedurine and accordingly provides exceptionally low incidence of leakagewhen worn by an infant in the normal manner. Similar results can beobtained if air-laid stiffened fibers are substituted for the wet-laidstiffened fibers in the acquisition layer of the absorbent core.

EXAMPLE III

This example illustrates the benefit of using relatively lowpolymerization/cure temperatures to consistently reduce coalescense ofwater droplets in the HIPE emulsion and to consistently obtain collapsedpolymeric foams according to the present invention, especially whenSPAN® 20 is the only emulsifier used in preparing the HIPE emulsion. Inthis example, a HIPE emulsion is prepared by a procedure similar to thatof Example I, but using a different lot of SPAN® 20 as the emulsifier.This HIPE emulsion is collected in a number of 1 pint plastic liddedjars. Four ovens are set at temperatures of 100° F. (37.8° C.), 115° F.(46.1° C.), 130° F. (54.4° C.), 150° F. (65.6°° C.), and several of thejars of HIPE emulsion are cured in each oven for 24 hours. A portion ofthe resultant cured foams are then washed with water and dried.Photomicrographs of the dried foams are then taken at magnifications of50×, 500× and 1000×. FIGS. 3a through 3d are representative of suchphotomicrographs (500× magnification). Specifically, FIG. 3a shows a cutsection of the 100° F. (37.8° C.) cured foam, FIG. 3b shows a cutsection of the 115° F. (46.1° C.) cured foam, FIG. 3c shows a cutsection of the 130° F. (54.4° C.) cured foam, and FIG. 3d shows a cutsection of the 150° F. (65.6°° C.) cured foam.

As shown in FIGS. 3a and 3b, the HIPE emulsions cured at lowertemperatures (i.e. below about 50° C.) resulted in foam structureshaving smaller pores of relatively uniform size, i.e. the foamstructures are relatively homogeneous. This suggests that there isreduced coalescence of the water droplets during curing of the HIPEemulsions. By contrast, as shown in FIGS. 3c and 3d, the HIPE emulsionscured at higher temperatures (i.e. above about 50° C.), resulted in foamstructures having numerous larger pores and a relatively nonuniform poresize, i.e. the foam structures are essentially heterogeneous. Thissuggests significantly increased coalescence of the water dropletsduring curing of the HIPE emulsions.

The homogeneity of these cured foams in terms of cell sizes andcoalescence can be graded qualitatively from the photomicrographs usingthe following scale:

    ______________________________________    Grade   Description    ______________________________________    1       massive coalescence, barely recognizable as foam    2       very bad coalescence, open voids, smeared struts    3       bad coalescence, thickened struts    4       moderate coalescence, some strut anomalies (thickened)    5       minor coalescence, no strut anomalies    6       minimal coalescence, no strut anomalies    7       well resolved homogeneous structure    ______________________________________

Several determinations are made for each of the cured foams based on theabove scale, and are then averaged to obtain a mean grade for each ofthe cured foams, as shown in the following table:

    ______________________________________    Cure Temperature                    Mean Grade    ______________________________________    100° F. (37.8° C.)                    6.5    115° F. (46.1° C.)                    5.5    300° F. (54.4° C.)                    3.3    150° F. (65.6° C.)                    2.5    ______________________________________

A portion of the above foam samples cured at 115° F. (46.1° C.), 130° F.(54.40° C.), and 150° F. (65.6° C.), are also evaluated for the abilityto remain thin after washing with a salt solution, followed by pressingand drying. The cured foams are processed by washing with an aqueoussolution of 1.0% calcium chloride, pressing to remove some of the water,and then oven drying (i.e. at about 150° F., 65.6° C.). Each of thedried foams is evaluated to determine whether it maintained itscollapsed state, i.e. 10-30% of its original expanded thickness. Thedried foam samples cured at 130° F. (54.4° C.), and 150° F. (65.6° C.)did not remain thin. By contrast, the dried foam samples cured at 115°F. (46.1° C.) did remain thin.

EXAMPLE IV

The following examples illustrate the preparation of HIPE foams usingsorbitan laurate (SPAN® 20) and polyglycerol fatty acid ester (PGE) orsorbitan palmitate (SPAN® 40) co-emulsifier systems:

EXAMPLE IV A

Anhydrous calcium chloride (36.32 kg) and potassium persulfate (568 g)are dissolved in 378 liters of water. This provides the water phasestream used in forming the HIPE emulsion.

To a monomer mixture comprising styrene (1600 g), divinylbenzene (55%technical grade, 1600 g), and 2-ethylhexylacrylate (4800 g) is addedsorbitan laurate (960 g as SPAN® 20). To another half-size batch of thesame monomer mixture is added PGE emulsifier (480 g) that imparts aminimum oil/water IFT of 0.09 dynes/cm. This PGE is obtained byesterifying polyglycerols with fatty acids in a weight ratio of 64:36using sodium hydroxide as the catalyst at 210° C. under conditions ofmechanical agitation, nitrogen sparging and gradually increasing vacuum,with subsequent phosphoric acid neutralization, cooling to about 60° C.,and settling to reduce unreacted polyglycerol. The composition of thepolyglycerols and fatty acids used in making the PGE are shown in thefollowing table:

    ______________________________________                  Wt. %    ______________________________________    Polyglycerols    linear diglycerols                    63.5    triglycerol     36.0    or higher    cyclic diglycerols                    0.4    Fatty Acids    C8              --    C10             --    C12             31.7    C14             37.2    C16             11.5    C18:0           3.2    C18:1           13.8    C18:2           1.5    ______________________________________

After mixing, each oil phase batch is allowed to settle overnight. Thesupernatant is withdrawn from each batch and mixed at a ratio of 2 partsof the SPAN® 20 containing oil phase to 1 part of the PGE containing oilphase. (About 75 g of a sticky residue is discarded from each of thebatches.)

At an aqueous phase temperature of 43° to 45° C. and an oil phasetemperature of 22° C., separate streams of the oil and water phases arefed to a dynamic mixer in the form of a pin impeller. This pin impellerhas a cylindrical shaft of about 21.6 cm in length with a diameter ofabout 1.9 cm. The shaft holds 4 rows of pins, two rows having 17 pinsand two rows having 16 pins, each pin having a diameter of 0.5 cm andextending outwardly 1.6 cm from the central axis of the shaft. The pinimpeller is mounted within a cylindrical sleeve with the pins having aclearance of 0.8 mm from the inner wall.

A spiral static mixer (14 in. long by 1/2 in. outside diameter, TAHIndustries Model 070-821, modified by cutting off 2.4 inches) is mounteddownstream from the dynamic mixer to provide back pressure in thedynamic mixer and to provide uniformity in the HIPE emulsion. Thecombined dynamic and static mixer apparatus is filled with oil and waterphases at a ratio of 2 parts water to 1 part oil. The apparatus isvented to allow air to escape until filling of the apparatus iscomplete. The flow rates during filling are 3.0 g/sec oil phase and 4.5cc/sec water phase.

Once the apparatus is filled, agitation is begun, with the impellerturning at 1100 RPM. The aqueous phase flow rate is then evenly rampedup to 46.5 cc/sec and the oil phase flow rate is evenly ramped down to1.77 g/sec over a time period of 120 sec. The back pressure created bythe dynamic and static mixers at this point is 4.9 psi. Over a timeperiod of 30 sec, the impeller is slowed to 1000 RPM. When the backpressure drops to approximately 3 psi, the impeller speed is theninstantly increased to 1800 RPM, and the back pressure increased to 5.5psi. The water and oil flows are then adjusted to 47.8 cc/sec and 1.66g/sec, respectively.

The HIPE emulsion is collected in molds (Rubbermaid Economy Cold FoodStorage Boxes made of food grade polyethylene, Model 3500), havinginside dimensions of 15 in. by 23 in. by 9 in. deep. The molds arepretreated with a film of a solution comprising 20% SPAN® 20 in xylenewhich had been allowed to settle overnight to remove insolubles. Themolds are preheated to facilitate the evaporation of the xylene andleave behind only the SPAN® 20. 47 liters of HIPE emulsion are collectedin each mold. The filled molds are kept in a room maintained at 65° C.for 18 hours to allow for curing. The cured foam is then washed with a1% calcium chloride solution. The residual solution retained in the foambefore drying is 5 times the weight of foam.

FIG. 4 is a photomicrograph (1000× magnification) that shows arepresentative polymeric foam prepared from a HIPE emulsion using aSPAN® 20/PGE coemulsifier system like that of this example. The foamstructure shown in FIG. 4 is in its expanded state. As can be seen, thisfoam structure has relatively small pores of relatively uniform size,i.e. the foam structure is relatively homogeneous. This suggestsevidence of reduced coalescence of the water droplets during curing ofthe HIPE emulsion.

EXAMPLE IV B

A water phase containing calcium chloride/potassium persulfate and anoil phase monomer mixture containing SPAN® 20 are prepared as in ExampleIV A. A half-size batch monomer mixture is also prepared by adding a PGEemulsifier that imparts a minimum oil/water IFT of 0.22 dynes/cm. ThisPGE is obtained by esterifying polyglycerols with fatty acids in aweight ratio of about 67:33 using reaction conditions similar to thosedescribed in Example IV A. The composition of the polyglycerols andfatty acids used in making the PGE are shown in the following table:

    ______________________________________                  Wt. %    ______________________________________    Polyglycerols    linear diglycerols                    73.1    triglycerol     24.5    or higher    cyclic diglycerols                    2.4    Fatty Acids    C8              --    C10             --    C12             32    C14             37    C16             11    C18:0           3.2    C18:1           13    C18:2           1.5    ______________________________________

After mixing, each oil phase batch is allowed to settle overnight withthe supernatants being withdrawn and mixed at a ratio of 2 parts of theSPAN® 20 containing oil phase to 1 part PGE containing oil phase, as inExample IV A. The aqueous and oil phases are then fed to the combineddynamic and static mixer apparatus as in Example IV A. The combinedapparatus is filled with the oil and water phases at a ratio of 2 partswater to 1 part oil, while venting the apparatus to allow air to escapeuntil filling of the apparatus is complete. The flow rates duringfilling are 3.0 g/sec oil phase and 7.5 cc/sec water phase.

Once the apparatus is filled, agitation is begun, with the impellerturning at 1200 RPM. The aqueous phase flow rate is then evenly rampedup to 45.0 cc/sec and the oil phase flow rate is evenly ramped down to1.66 g/sec over a time period of 60 sec. The impeller RPM is thenlowered to 1100 evenly over a period of 30 sec and then instantlyincreased to 1800 RPM. The water phase flow rate is then adjusted to47.6 cc/sec. The back pressure is 5.3 psi. The formed emulsion iscollected in molds and then kept in a room maintained at 65° C. for 18hours to allow for curing, as in Example IV A. The cured foams arewashed with a 1% calcium chloride solution. The residual solutionretained in the foam before drying is 5 times the weight of foam.

EXAMPLE IV C

A water phase containing calcium chloride/potassium persulfate and anoil phase monomer mixture containing SPAN® 20 are prepared as in ExampleIV A. A half-size batch monomer mixture is also prepared by adding a PGEemulsifier that imparts a minimum oil/water IFT of 0.08 dynes/cm. ThisPGE is obtained by esterifying polyglycerols with fatty acids in aweight ratio of about 67:33 using reaction conditions similar to thosedescribed in Example IV A. The composition of the polyglycerols andfatty acids used in making the PGE are shown in the following table:

    ______________________________________                  Wt. %    ______________________________________    Polyglycerols    linear diglycerols                    -71    triglycerol     -24    or higher    cyclic diglycerols                    -5    Fatty Acids    C8              --    C10             4.4    C12             43.6    C14             25.1    C16             12.1    C18:0           3.8    C18:1           9.2    C18:2           1.4    ______________________________________

After mixing, each oil phase batch is allowed to settle overnight withthe supernatants being withdrawn and mixed at a ratio of 2 parts of theSPAN® 20 containing oil phase to 1 part PGE containing oil phase, as inExample IV A. The aqueous (45°-47° C.) and oil phases are then fed tothe combined dynamic and static mixer apparatus as in Example IV A. Thecombined apparatus is filled with the oil and water phases at a ratio of2 parts water to 1 part oil, while venting the apparatus to allow air toescape until filling of the apparatus is complete. The flow rates duringfilling are 2.2 g/sec oil phase and 4.7 cc/sec water phase.

Once the apparatus is filled, agitation is begun, with the impellerturning at 1800 RPM. The aqueous phase flow rate is then evenly rampedup to 45.5 cc/sec and the oil phase flow rate is evenly ramped down to1.59 g/sec over a time period of 90 sec. The back pressure is 5.4 psi.The HIPE emulsion is collected in molds and then kept in a roommaintained at 65° C. for 18 hours to allow for curing, as in Example IVA. The cured foams are washed with a 1% calcium chloride solution. Theresidual solution retained in the foam before drying is 5 times theweight of foam.

EXAMPLE IV D

A water phase containing calcium chloride/potassium persulfate and anoil phase monomer mixture containing SPAN® 20 are prepared as in ExampleIV A. A half-size batch monomer mixture is also prepared by adding a PGEemulsifier that imparts a minimum oil/water IFT of 0.013 dynes/cm. ThisPGE is obtained by esterifying polyglycerols with fatty acids in aweight ratio of about 61:39 using reaction conditions similar to thosedescribed in Example IV A. The composition of the polyglycerols andfatty acids used in making the PGE are shown in the following table:

    ______________________________________                  Wt. %    ______________________________________    Polyglycerols    linear diglycerols                    -15    triglycerol     -85    or higher    cyclic diglycerols                    --    Fatty Acids    C8              6    C10             5    C12             55    C14             23    C16             6    C18:0           3    C18:1           1    C18:2           1    ______________________________________

After mixing, each oil phase batch is allowed to settle overnight withthe supernatants being withdrawn and mixed at a ratio of parts of theSPAN® 20 containing oil phase to 1 part PGE containing oil phase, as inExample IV A. The aqueous (55°-60° C.) and oil phases are then fed tothe combined dynamic and static mixer apparatus as in Example IV A. Thecombined apparatus is filled with the oil and water phases at a ratio of2 parts water to 1 part oil, while venting the apparatus to allow air toescape until filling of the apparatus is complete. The flow rates duringfilling are 3.0 g/sec oil phase and 6.0 cc/sec water phase.

Once the apparatus is filled, agitation is begun, with the impellerturning at 1800 RPM. The aqueous phase flow rate is then evenly rampedup to 45.7 cc/sec and the oil phase flow rate is evenly ramped down to1.58 g/sec over a time period of 120 sec. The back pressure is 5.4 psi.The HIPE emulsion is collected in molds and then kept in a roommaintained at 65° C. for 18 hours to allow for curing, as in Example IVA. The cured foams are washed with a 1% calcium chloride solution. Theresidual solution retained in the foam before drying is 5 times theweight of foam.

EXAMPLE IV E

A water phase containing calcium chloride/potassium persulfate and anoil phase monomer mixture containing SPAN® 20 are prepared as in ExampleIV A. A half-size batch monomer mixture is also prepared by adding a PGEemulsifier that imparts a minimum oil/water IFT of 0.042 dynes/cm. ThisPGE is obtained by esterifying polyglycerols with fatty acids in aweight ratio of about 67:33 using reaction conditions similar to thosedescribed in Example IV A. The composition of the polyglycerols andfatty acids used in making the PGE are shown in the following table:

    ______________________________________                  Wt. %    ______________________________________    Polyglycerols    linear diglycerols                    70.6    triglycerol     24.1    or higher    cyclic diglycerols                    5.3    Fatty Acids    C8              --    C10             --    C12             32.1    C14             38.6    C16             11    C18:0           3.2    C18:1           13.4    C18:2           1.4    ______________________________________

After mixing, each oil phase batch is allowed to settle overnight withthe supernatants being withdrawn and mixed at a ratio of 2 parts of theSPAN® 20 containing oil phase to 1 part PGE containing oil phase, as inExample IV A. The aqueous and oil phases are then fed to the combineddynamic and static mixer apparatus as in Example IV A. The combinedapparatus is filled with the oil and water phases at a ratio of 2 partswater to 1 part oil, while venting the apparatus to allow air to escapeuntil filling of the apparatus is complete. The flow rates duringfilling are 1.7 g/sec oil phase and 3.0 cc/sec water phase.

Once the apparatus is filled, agitation is begun, with the impellerturning at 1100 RPM. The aqueous phase flow rate is then evenly rampedup to 48.4 cc/sec. over a time period of 90 sec. The back pressure is5.0 psi. The impeller RPM is then instantly increased to 1800 RPM. Theback pressure increases to 5.8 psi. The HIPE emulsion is collected inmolds and then kept in a room maintained at 65° C. for 18 hours to allowfor curing, as in Example IV A. The cured foams are washed with a 1%calcium chloride solution. The residual solution retained in the foambefore drying is 5 times the weight of foam.

EXAMPLE IV F

A water phase containing calcium chloride/potassium persulfate and anoil phase monomer mixture containing SPAN® 20 are prepared as in ExampleIV A. To the monomer mixture is added sorbitan laurate (480 g as SPAN®20) and a mixture of sorbitan laurate (240 g) and sorbitan palmitate(240 g as SPAN® 40). After mixing, the oil phase is allowed to settleovernight, with the supernatant being withdrawn for use in forming theHIPE emulsion.

The aqueous (48°-50° C.) and oil phases are then fed to the combineddynamic and static mixer apparatus as in Example IV A. The combinedapparatus is filled with the oil and water phases at a ratio of 2 partswater to 1 part oil, while venting the apparatus to allow air to escapeuntil filling of the apparatus is complete. The flow rates duringfilling are 3.0 g/sec oil phase and 6 cc/sec water phase.

Once the apparatus is filled, agitation is begun, with the impellerturning at 1800 RPM. The aqueous phase flow rate is then evenly rampedup to 42.3 cc/sec and the oil phase flow rate is evenly ramped down to1.5 g/sec over a time period of 60 sec. The back pressure is 4.5 psi.The HIPE emulsion is collected in molds (round tubs with a central core)and then kept in a room maintained at 65° C. for 18 hours to allow forcuring, as in Example IV A. The cured foams are washed with a 1% calciumchloride solution. The residual solution retained in the foam beforedrying is 5 times the weight of foam.

What is claimed is:
 1. In a process for the preparation of a collapsed,but expandable, absorbent polymeric foam material which comprises thesteps of:A) forming a water-in-oil emulsion from:1) an oil phasecomprising:a) from about 67 to about 98% by weight of a monomercomponent comprising:i) from about 5 to about 40% by weight of asubstantially water-insoluble, monofunctional glassy monomer; ii) fromabout 30 to about 80% by weight of a substantially water-insoluble,monofunctional rubbery comonomer; iii) from about 10 to about 40% byweight of a substantially water-insoluble, polyfunctional crosslinkingagent, and b) from about 2 to about 33% by weight of an emulsifiercomponent which is soluble in the oil phase and which is suitable forforming a stable water-in-oil emulsion; and 2) a water phase comprisingan aqueous solution containing from about 0.2 to about 20% by weight ofa water-soluble electrolyte; 3) the weight ratio of the water phase tothe oil phase being in the range of from about 12:1 to about 100:1; B)polymerizing the monomer component in the oil phase of the water-in-oilemulsion to form a polymeric foam material; and C) dewatering thepolymeric foam material to an extent such that a collapsed, polymericfoam material is formed that will re-expand upon contact with aqueousbody fluids, the improvement which comprises carrying out said emulsionformation and polymerization steps (A) and (B) in a manner such thatcoalescence of the water droplets formed in the water-in-oil emulsion isreduced so that the number average cell size of the polymeric foammaterial is about 50 microns or less.
 2. The process of claim 1 whereinthe water phase further comprises from about 0.02 to about 0.4% byweight of a water-soluble free radical polymerization initiator, andwherein the weight ratio of water phase to oil phase is from about 20:1to about 70:1.
 3. The process of claim 2 wherein:1) the oil phasecomprises:a) from about 80 to about 95% by weight monomer componentcomprising:i) from about 10 to about 30% by weight glassy monomerselected from the group consisting of styrene-based monomers andmethacrylate-based monomers; ii) from about 50 to about 70% by weightrubbery comonomer selected from the group consisting of n-butylacrylateand 2-ethylhexylacrylate; iii) from about 15 to about 25% by weightpolyfunctional crosslinking agent selected from the group consisting ofdivinylbenzene divinyltoluene and diallylphthalate; and b) from about 5to about 20% by weight emulsifier component comprising sorbitan laurate;2) the water phase comprises from about 1 to about 10% by weight calciumchloride; 3) the weight ratio of water phase to oil phase is from about25:1 to about 50:1.
 4. The process of claim 3 wherein the monomercomponent comprises:i) from about 15 to about 25% by weight styrene; ii)from about 55 to about 65% by weight 2-ethylhexylacrylate; iii) fromabout 15 to about 25% by weight divinylbenzene.
 5. The process of claim2 wherein polymerization step (B) is carried out at a temperature offrom about 30° to about 50° C. for from about 4 to about 24 hours. 6.The process of claim 5 wherein polymerization step (B) is carried out ata temperature of from about 35° to about 45° C. for from about 4 toabout 18 hours.
 7. The process of claim 2 wherein the emulsifiercomponent comprises sorbitan laurate and polyglycerol fatty acid esterin a weight ratio of from about 10:1 to about 1:10, the polyglycerolester being derived from: (1) a polyglycerol having a linear diglycerollevel of at least about 60% by weight, a tri- or higher polyglycerollevel of no more than about 40% by weight, and a cyclic diglycerol levelof no more than about 10% by weight; (2) a fatty acid reactant having afatty acid composition wherein the combined level of C₁₂ and C₁₄saturated fatty acids is at least about 40%, the level of C₁₆ saturatedfatty acid is no more than about 25%, the combined level of C₁₈ orhigher saturated fatty acids is no more than about 10%, and the combinedlevel of C₁₀ or lower fatty acids is no more than about 10%; (3) theweight ratio of polyglycerol (1) to fatty acid reactant (2) being fromabout 50:50 to about 70:30.
 8. The process of claim 7 wherein the weightratio of sorbitan laurate to polyglycerol ester is from about 4:1 toabout 1:1, and wherein the polyglycerol ester is derived from: (1) apolyglycerol having a linear diglycerol level of from about 60 to about80% by weight, and a tri- or higher polyglycerol level of from about 20to about 40% by weight; (2) a fatty acid reactant having a fatty acidcomposition wherein the combined level of C₁₂ and C₁₄ saturated fattyacids is at least about 65%, the level of C₁₆ saturated fatty acid is nomore than about 15%, the combined level of C₁₈ or higher fatty acids isno more than about 4%, and the combined level of C₁₀ or lower fattyacids is no more than about 3%; (3) the weight ratio of polyglycerol (1)to fatty acid reactant (2) being from about 60:40 to about 70:30.
 9. Theprocess of claim 7 wherein the polyglycerol ester imparts a minimumoil/water interfacial tension at 50° C. of at least about 0.06 dynes/cm.10. The process of claim 9 wherein the polyglycerol ester imparts aminimum oil/water interfacial tension at 50° C. of from about 0.09 toabout 0.3 dynes/cm.
 11. The process of claim 7 wherein polymerizationstep (B) is carried out at a temperature of above about 50° C.